Use namespacing conventions in WasmContext.

The competition is a bit tight, but WasmContext manipulates more IR
concepts than Wasm contexts. Therefore, we import IR things without
namespace, and Wasm things under short namespaces. We use `wa`,
`watpe`, `wamod` and `wanme` by analogy to the compiler backend and
JS backend conventions, which use `js` and `jsnme`.

Author: sjrd