Add ArgIteratorWasi and integrate it with ArgIterator
Add ArgIteratorWasi and integrate it with ArgIterator

This commit pulls WASI specific implementation of args extraction
from the runtime from `process.argsAlloc` and `process.argsFree`
into a new iterator struct `process.ArgIteratorWasi`. It also
integrates the struct with platform-independent `process.ArgIterator`.

raulgrell/zig,raulgrell/zig,zig-lang/zig,zig-lang/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,andrewrk/zig,raulgrell/zig,andrewrk/zig