This currently creates a field which is always false on GenericParamDefKind for future use when
consts are permitted to have defaults

Update const_generics:default locations

Previously just ignored them, now actually do something about them.

Fix using type check instead of value

Add parsing

This adds all the necessary changes to lower const-generics defaults from parsing.

Change P<Expr> to AnonConst

This matches the arguments passed to instantiations of const generics, and makes it specific to
just anonymous constants.

Attempt to fix lowering bugs

StructField -> FieldDef ("field definition")
Field -> ExprField ("expression field", not "field expression")
FieldPat -> PatField ("pattern field", not "field pattern")

Also rename visiting and other methods working on them.

The attributes for statements and those of the statements' content.

Fixes some punctuation and formatting; also makes some small wording changes.

Crate root is sufficiently different from `mod` items, at least at syntactic level.

Also remove customization point for "`mod` item or crate root" from AST visitors.

Items are guaranteed to be HIR owner.

Also switches on formatting of the mir build module

These fields are only used by hygiene serialized, and should not be
accessed by anything outside of `rustc_span`.

- Adds optional default values to const generic parameters in the AST
  and HIR
- Parses these optional default values
- Adds a `const_generics_defaults` feature gate