[WebAssembly] Implement a prototype instruction encoder and disassembler.

This is using an extremely simple temporary made-up binary format, not the
official binary format (which isn't defined yet).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257440 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed