feat: Add iterators and lists

[mark-koch]

Adds iterators, lists, for loops, and list comprehensions.

This PR sets up the basics:

• Add new AST nodes for desugared list comprehensions and iterator commands (in nodes.py)
• Add ListType and LinstType to the Guppy type hierarchy (in gtypes.py)
• Declare list methods in the builtins module (in prelude/builtins.py)
• Store/compute Hugr type bounds for guppy types

The remaining functionality is added in the following child PRs:

• feat: Build for loops and comprehensions #68
• feat: Type check lists and comprehensions #69
• feat: Compile lists and comprehensions #70
• test: Add list, loop, and comprehension tests #71

[ss2165]

OOI what's the logic here? Try to transform the list and if not just transform the element?

[mark-koch]

Yes, if the transformer doesn't specify what to do with a type, we leave the outer type as is and recurse

[ss2165]

why return Iterator rather than just Iterable? (and then avoid the call to iter())

[mark-koch]

Good point. I think I initially wanted an iterator since I wanted to chain them at some point, but didn't turn out to be necessary.

I created issue #75 for this

[ss2165]

could just

Suggested change

	res = TypeBound.Any
	return TypeBound.Any

[mark-koch]

In fact, the following should be enough?

res = TypeBound.Eq
for b in bs:
    if b == TypeBound.Any:
        return TypeBound.Any
    if res == TypeBound.Eq:
        res = b
return res

[ss2165]

what's the difference between guppy.type and guppy.extend_type ?

[mark-koch]

guppy.type declares a new type, wheras guppy.extend_type takes an exisisting type and allows users to define new methods on them (similar to extension methods in e.g. C#)