Typed Gerbil

[vyzo]

It is alive!

TODO:

• [x] compiler understands expression types and performs static type checking and type directed optimizations
• [x] bootstrap
• [x] move contract and interface macros to the core prelude
• [x] typed gerbil prelude
• [x] port runtime from the defapi macro to the typed gerbil prelude
• [x] bootstrap
• [x] make stdlib build
• [x] port select stdlib modules to typed gerbil

Follow up:

• [ ] port more stdlib modules to typed gerbil
• [ ] documentation!
• [ ] more tests

Typed Gerbil

This pr introduces the first iteration of Typed Gerbil. The compiler now understands and can infer expression types with MOP classes and interfaces, and it can perform static type checks and type directed performance optimization.

Notable linguistic changes:

• contracts and interface macros move to the core prelude
• core prelude exports typed definition forms
• underscores are no longer distinguished as null bindings; this was an interesting idea, but it causes problems in practice (it complicates expansion) and it is also incompatible with standard scheme semantics where no identifier is privileged.

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[vyzo]

rebased on master, this is ready to merge once we get the green ticks.