[wasm] Clean up wasm module implementation

By defining functions with namespace prefix, the compiler checks that
they were previously declared, and checks that the signature matches.
I stumbled across this several times when changing the interface of a
function in the header.
With this change you get a compile error right away instead of a linker
error in the very end.

This change also revealed two functions which could be placed in an
anonymous namespace, saving 5.5kB program size in Debug build, 2.3kB in
Optdebug and 0.3kB in Release. It's also opening more options for
compiler optimizations, as the functions now have internal linkage.

R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2415603002
Cr-Commit-Position: refs/heads/master@{#40233}