faster calling convention

[nikodemus]

• Methods set up their own contexts, and receive the arguments as C-arguments.
• Methods which only send a single message don't need to allocate a context at all, but can use their sender context.
• Builtin methods can be additionally annotated as dynamicExtent, meaning their context can be stack-allocated, or noContext if they don't need a context at all. (Assumption being the compiler generates the context-allocating code for them automatically.)

[nikodemus]

Partially done on dev branch.

[nikodemus]

Stale thanks to the new runtime model and backend.