Modules

[PaulBone]

Support multi-module programs and clarify the builtin module.

Depends on #52, #77 and #178

[PaulBone]

I think the plan will be something like

Bytecode level:

• [x] Make scaffolding for packages (#229)
• [x] Multi! Module loading (#158)
• [x] Linking (#219)

Compiler machinary:

• [x] Reading interface files (#246)
• [x] Writing interface files (#247)
• Transitive info in interface files out of scope.

Language syntax features:

• [x] Export directives for functions
• Module local state and resources (out of scope)
• [ ] Export types, constructors and accessors separately
• [ ] Selective import

Building

• [x] Separate compilation
• [ ] Project file support (#253)
• [x] Change main function (#245)
• [ ] Find interfaces from libraries / package libraries?