Fix the grammar of const param defaults by reusing rule for const args

[fmease]

struct T<const N: U = crate>; is syntactically valid but the previous definition of rule ConstParam didn't capture that fact since it only listed IDENTIFIER while it needed to be SimplePathSegment.

[fmease]

const arg / constant argument is the term we use in rustc (internally and in diagnostics).

Moreover, in rustc's parser, const arg doesn't just refer to const arguments literally but also to const param defaults (and to the RHS of associated const bindings as in TraitRef<CT = { 1 + 1 }> but that's unstable syntax). So "reusing" ConstArg in rule ConstParam doesn't come out of thin air.

View changes since the review

[fmease]

I found an issue: The "const arg" used for the param default should include Self while the "const arg proper" should not contain Self as that would conflict with type arguments. Let me think about how to solve this most nicely.

Edit: Well, as a matter of fact, GenericArgsConst already conflicts with Type on master ... I think remember a Zulip or GH discussion where ehuss (?) stated that it's still unclear how we want to represent identifier const args that can refer to either type or const in the grammar.