[bindings] Handle generic-ized impl blocks by concretizing them

[bindings] Handle generic-ized impl blocks by concretizing them

This handles, for example, the `impl<X: Y> for Features<X>` blocks
which are generic across a number of different contexts. We do so
by walking the set of structs which alias Features and then walking
their generic arguments to check that they meet the bounds
specified in the impl block. For each alias which does, we create
a dummy, explicit, `impl XFeatures` block with the same content as
the original and recurse.