build: global Function() and SignatureExpr() (#215)

Exposes a few functions on Context that are not available globally. This
oversight suggests that #33 or #133 would be reasonable. `Signature()` is
currently excluded because it causes a conflict for users who have
dot-imported both `build` and `gotypes`. For example:

https://github.com/segmentio/asm/blob/18af27c3ce38682a2545f2d95e5ef564dd312d6e/build/base64/decode_asm.go#L13-L14
https://github.com/phoreproject/bls/blob/a88a5ae26844d7293359422888d7c7f69f43c845/asm/asm.go#L6-L7

Author: mmcloughlin

build/global.go

@@ -122,6 +122,9 @@ func Store(src reg.Register, dst gotypes.Component) { ctx.Store(src, dst) }
 // Dereference loads a pointer and returns its element type.
 func Dereference(ptr gotypes.Component) gotypes.Component { return ctx.Dereference(ptr) }
 
+// Function starts building a new function with the given name.
+func Function(name string) { ctx.Function(name) }
+
 // Doc sets documentation comment lines for the currently active function.
 func Doc(lines ...string) { ctx.Doc(lines...) }
 
@@ -131,6 +134,9 @@ func Pragma(directive string, args ...string) { ctx.Pragma(directive, args...) }
 // Attributes sets function attributes for the currently active function.
 func Attributes(a attr.Attribute) { ctx.Attributes(a) }
 
+// SignatureExpr parses the signature expression and sets it as the active function's signature.
+func SignatureExpr(expr string) { ctx.SignatureExpr(expr) }
+
 // Implement starts building a function of the given name, whose type is
 // specified by a stub in the containing package.
 func Implement(name string) { ctx.Implement(name) }