implement runtime

[KeenS]

Write GC

[KeenS]

emscripten adds unnecessarily imports so may need to implement it with (restricted) webml itself

[KeenS]

roadmap to implement the runtime:

1. [x] implement closure calls
2. [ ] implement fast call (de-closure) optimization
+ implies inlining, dead code optimization
3. [ ] implement intrinsics
4. [ ] implement dead link elimination
+ (opt) no_std or no_runtime declaration
5. [ ] implement GC in webml Rust.

[KeenS]

memo: to implement closure calls, make all the function calls closure calls. To do so, separate definition for closure calls and fast calls. TBD: generate closed function and fast function for each functions, or generate them on demand?

[KeenS]

memo: give marking functions instead of tag bits