Add the DescSet type

[timostamm]

Introduce the type DescSet, a set of descriptors with convenient accessors to look up descriptors by name.
This type is the successor to DescriptorSet and registries, and combines the features of both.

For example, if generated files exports the messages User and Foo, a set of the descriptors can be created with:

// before:
const reg = createRegistry( User, Foo );

// now:
const set = createDescSet( UserDesc, FooDesc );

To create a set of descriptors from a google.protobuf.FileDescriptorSet message:

declare const fileDescriptorSet: FileDescriptorSet;

// before:
const set = createDescriptorSet( fileDescriptorSet );

// now:
const set = createDescFileSet( fileDescriptorSet );

This creates a DescSet that also includes file descriptors, a DescFileSet. The old function createDescriptorSet() and the interface DescriptorSet are not removed by this PR, and continue to work as expected.

Descriptor sets can be merged by passing multiple input sets to the variadic function createDescSet() or createDescFileSet():

const a = createDescSet( UserDesc, FooDesc );
const b = createDescriptorSet( fileDescriptorSet );
const set = createDescSet( a, b );

This feature makes it trivial to create "registries" from multiple generated file descriptors. If a descriptor is given multiple times (identified by the same type name), the one given last wins.

[timostamm]

(This is based on #743, which needs to be merged first.)