[fidl][coding] Implement fidl decoding

This defines the interface to fidl encoding and decoding. This
includes the definitions of fidl types like fidl_string_t, and the
declarations of fidl_encode and fidl_decode.

This also implements and tests fidl_decode.

TO-487 #done

Change-Id: I7a5490c6efdd9c4ea045a9a1851030441bc72299