Redesign core

[b-studios]

Every few months/years we redesign core. It is again time.

There are some universally accepted requirements, some nice to haves, and some controversial things.

Universally Accepted

• move away from handlers to delimited control
• delimited control should ALWAYS be bidirectional
• reset binds a label / prompt, shift refers to a label.
• have separate invoke
• have separate make or similar for constructor calls

Nice to have

• group arguments and parameters in separate data type
• add syntactic category of "binder"
• track the original source location in the AST

Controversial

• shallow resumptions
• replace run by let x = run s; s
• also include expression level if, defs, with separate calling convention.
• what about subtyping? Where do we allow it?
• bound type variables as DeBruijn

[jiribenes]

Related existing issues: #331, #364