Implement existentially quantified parameters

[rachitnigam]

Fixes #121.

[rachitnigam]

Okay, basic parsing and astconv conversion is working. Next step is to separate the components into assume and assert. Previously, all component constraints were asserted into the calling component by now some of them should be assumed. This is because constraints over existentially quantified parameters are supplied by instances instead of being proven by the caller.

After that, we need to figure out what to do with monomorphization. For now, maybe I'll require that all existential parameters have defined values in the program and eventually remove that constraint in a future PR when we want to be able to do retiming.

[rachitnigam]

Okay, this PR now implements a basic AST to IR lowering for existential types. Monomorphization crashes when it sees existential parameters. A followup PR will build the flow that allows compilation of programs with existential types.

[rachitnigam]

This should probably not be reviewed till #304 and #305 are merged. Currently, the diff on this is pretty big because it includes those changes.

[rachitnigam]

Okay, this is ready to review now!