Adds a 'buildGo.proto' function which takes a single .proto file as
its source and generates a corresponding Go library which can then be
imported.
'proto' takes these arguments (Yants-style type definition):
struct "protoArgs" {
# required:
name = string;
proto = path;
# optional:
extraDeps = list goLib; # defaults to [ ]
protocFlags = option string;
}
Note that proto libraries will automatically have dependencies for the
required protobuf Go libraries added to them.
gRPC is not (yet) supported.
