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Dana Jansens
30b2b5ef81
When attempting to convert to a value of type FacetType, and the source
value is a FacetAccessType, then if the type of the underlying
FacetValue is the same as the target, we can use the FacetValue there as
the conversion output.
If the type of the FacetValue differs, then we still want to do impl
lookup with the FacetValue to see if it matches with the target
FacetType, but that is still a TODO.
This allows a generic function with a value whose type is constrained by
a FacetType (thus the value's type is a FacetAccessType), to call other
functions with the value as an argument when it is constrained by the
same FacetType:
```
fn F[T:! FacetType](x: T);
fn G[T:! FacetType](x: T) { F(x); }
```
It is also an optimization to avoid impl lookup where we've already done
it to produce the FacetAccessType.
Adds a bunch of new tests with values of types which are constrained by
a facet type (or "facet value value" for short), with some more tests
that currently fail and should be made to pass.