Introduce minimal WireSchema

This decouples human-readable Schema that is in JSON
format from WireSchema that is serialized in binary
format for efficiency. WireSchema contains only the
information that is necessary for wire compatibility
checks.

Resolves #24
(Note that we still need struct names for checks).

Here are byte size comparisons between JSON and WIRE
for the Otel/TEF schema with "Metrics" root:

```
JSON:  2976, zstd:  760
WIRE:   115, zstd:  113
```

go/grpc/client.go

@@ -28,7 +28,7 @@ type Client struct {
 	grpcClient   stef_proto.STEFDestinationClient
 	stream       stef_proto.STEFDestination_StreamClient
 	callbacks    ClientCallbacks
-	clientSchema *schema.Schema
+	clientSchema *schema.WireSchema
 	logger       types.Logger
 
 	// Running state
@@ -88,7 +88,7 @@ type ClientSettings struct {
 	Logger types.Logger
 	// gRPC stream to send data over.
 	GrpcClient   stef_proto.STEFDestinationClient
-	ClientSchema *schema.Schema
+	ClientSchema *schema.WireSchema
 	Callbacks    ClientCallbacks
 }
 
@@ -154,7 +154,7 @@ func (c *Client) Connect(ctx context.Context) (pkg.ChunkWriter, pkg.WriterOption
 	}
 
 	// Unmarshal server schema.
-	var serverSchema schema.Schema
+	var serverSchema schema.WireSchema
 	buf := bytes.NewBuffer(capabilities.Capabilities.Schema)
 	err = serverSchema.Deserialize(buf)
 	if err != nil {

go/grpc/server.go

@@ -124,7 +124,7 @@ type StreamServer struct {
 	stef_proto.UnimplementedSTEFDestinationServer
 
 	logger       types.Logger
-	serverSchema *schema.Schema
+	serverSchema *schema.WireSchema
 	maxDictBytes uint64
 	onStream     func(reader GrpcReader, ackFunc func(sequenceId uint64) error) error
 }
@@ -133,7 +133,7 @@ var _ stef_proto.STEFDestinationServer = (*StreamServer)(nil)
 
 type ServerSettings struct {
 	Logger       types.Logger
-	ServerSchema *schema.Schema
+	ServerSchema *schema.WireSchema
 	MaxDictBytes uint64
 	OnStream     func(reader GrpcReader, ackFunc func(sequenceId uint64) error) error
 }

go/otel/grpc_test.go

@@ -41,7 +41,7 @@ func TestGrpcWriteRead(t *testing.T) {
 
 	recordsReceivedAndVerified := make(chan struct{})
 	serverSettings := stefgrpc.ServerSettings{
-		ServerSchema: schema,
+		ServerSchema: &schema,
 		OnStream: func(source stefgrpc.GrpcReader, ackFunc func(sequenceId uint64) error) error {
 			reader, err := oteltef.NewMetricsReader(source)
 			require.NoError(t, err)
@@ -95,7 +95,7 @@ func TestGrpcWriteRead(t *testing.T) {
 
 	clientSettings := stefgrpc.ClientSettings{
 		GrpcClient:   grpcClient,
-		ClientSchema: schema,
+		ClientSchema: &schema,
 		Callbacks:    callbacks,
 	}
 	client := stefgrpc.NewClient(clientSettings)

go/otel/manual_test.go

@@ -284,7 +284,7 @@ func TestAnyValue(t *testing.T) {
 	}
 }
 
-func writeReadRecord(t *testing.T, withSchema *schema.Schema) *oteltef.Metrics {
+func writeReadRecord(t *testing.T, withSchema *schema.WireSchema) *oteltef.Metrics {
 	buf := &countingChunkWriter{}
 	writer, err := oteltef.NewMetricsWriter(buf, pkg.WriterOptions{Schema: withSchema})
 	require.NoError(t, err)
@@ -321,23 +321,21 @@ func TestWriteOverrideSchema(t *testing.T) {
 	assert.EqualValues(t, 4.5, readRecord.Point().Value().Float64())
 
 	// Write/read using full, unmodified schema
-	readRecord = writeReadRecord(t, schem)
+	readRecord = writeReadRecord(t, &schem)
 	assert.EqualValues(t, "abc", readRecord.Metric().Name())
 	assert.EqualValues(t, "scope", readRecord.Scope().Name())
 	assert.EqualValues(t, 123, readRecord.Point().Timestamp())
 	assert.EqualValues(t, oteltef.PointValueTypeFloat64, readRecord.Point().Value().Type())
 	assert.EqualValues(t, 4.5, readRecord.Point().Value().Float64())
 
 	// Remove "Monotonic" field (field #8) from "Metric" struct in the schema.
-	metric := schem.Structs["Metric"]
-	metric.Fields = metric.Fields[:7]
+	schem.StructFieldCount["Metric"] = 7
 
 	// Remove "Float64" field (field #2) from "PointValue" oneof struct in the schema.
-	pointValueOneOf := schem.Structs["PointValue"]
-	pointValueOneOf.Fields = pointValueOneOf.Fields[:1]
+	schem.StructFieldCount["PointValue"] = 1
 
 	// Write/read using reduced schema
-	readRecord = writeReadRecord(t, schem)
+	readRecord = writeReadRecord(t, &schem)
 	assert.EqualValues(t, "abc", readRecord.Metric().Name())
 	assert.EqualValues(t, "scope", readRecord.Scope().Name())
 	assert.EqualValues(t, 123, readRecord.Point().Timestamp())
@@ -350,11 +348,10 @@ func TestWriteOverrideSchema(t *testing.T) {
 	assert.EqualValues(t, 0.0, readRecord.Point().Value().Float64())
 
 	// Remove the entire "Point" field (field #6) from "Record" struct in the schema.
-	mainStruct := schem.Structs["Metrics"]
-	mainStruct.Fields = mainStruct.Fields[:5]
+	schem.StructFieldCount["Metrics"] = 5
 
 	// Write/read using reduced schema
-	readRecord = writeReadRecord(t, schem)
+	readRecord = writeReadRecord(t, &schem)
 	assert.EqualValues(t, "abc", readRecord.Metric().Name())
 	assert.EqualValues(t, "scope", readRecord.Scope().Name())
 	// All Point fields are default values because Point field was not encoded by Writer.