First-class impls?

[osa1]

Here's an alternative fix to #261: we make impls first-class and allow users to create and manipulate them.

• Conceptually, an impl block becomes a record of functions. (i.e. dictionaries)
• When calling a generic function with predicates, we implicitly synthesize and pass the records.
• However we also allow user to pass them explicitly, probably using the type application syntax.
• We also allow manipulating existing impls and creating new ones.
• When compiling generic functions with predicates, we monomorphise based on the impls passed (same as today).

When compiling explicit impl passing, we have two options:

1. We may have a notion of "const" expressions (like in most languages these days) and require the record exprs to be consts.
2. We may monomorphise the function to take a record object in runtime and call the functions in the record.

I think we may need something like (2) regardless, to support trait objects. So maybe we implement that first and then reuse.

Lots of details to figure out, but here's a simple example:

# In the standard library:
impl[ToDoc[t]] ToDoc[Vec[t]]:
    toDoc(self: Vec[t]) Doc:
        ...


# In a third-party library:
debug[ToDoc[t]](...):
    ...

Then in my code I want to use debug on a Vec[U32], but print the numbers in hex. I need a record for ToDoc[Vec[U32]] that will call something else other than toDoc on the elements. This will require a new syntax. Note that I may not have access to Vec's toDoc function here. I want to reuse it here, just overriding ToDoc[t] parts.

Just an idea:

debug[ToDoc[Vec[U32]] with ToDoc[U32] = (toDoc = toHexDoc)](myVec)

The (toDoc = toHexDoc) part is the record. with syntax specifies the predicate on the impl, in impl[ToDoc[t]] ToDoc[Vec[t]]: ....

Another idea: we allow naming impl blocks and have another ToDoc[U32] that prints in hex. Then use that named impl:

impl ToDoc[U32] as U32ToDocInHex:       # `as` gives name to the `impl`
    ...


debug[ToDoc[Vec[U32]] with U32ToDocInHex](...)

Here ToDoc[Vec[U32]] with U32ToDocInHex synthesizes a new impl for ToDoc[Vec[U32]] using U32ToDocInHex for ToDoc[U32], instead of the default one.

[osa1]

We should update the title: we can't make impls truly first-class, we just want to update the ergonomics a little bit and give some more flexibility as described above.