gccrs: ast: Move rust-path implementation out of rust-ast

gccrs: ast: Move rust-path implementation out of rust-ast

Move rust-path implementation details out of rust-ast.cc file. This will
hopefully decrease compile time and make things clearer.

gcc/rust/ChangeLog:

	* Make-lang.in: Add new file for rust-path implementation.
	* ast/rust-ast.cc (PathInExpression::as_string): Move function
	to the new file.
	(PathPattern::as_string): Likewise.
	(QualifiedPathInExpression::as_string): Likewise.
	(ConstGenericParam::as_string): Likewise.
	(QualifiedPathInType::as_string): Likewise.
	(TypePath::as_string): Likewise.
	(PathPattern::convert_to_simple_path): Likewise.
	(TypePath::as_simple_path): Likewise.
	(PathExprSegment::as_string): Likewise.
	(GenericArgs::as_string): Likewise.
	(GenericArgsBinding::as_string): Likewise.
	(TypePath::to_trait_bound): Likewise.
	(TypePathSegmentGeneric::as_string): Likewise.
	(TypePathFunction::as_string): Likewise.
	(TypePathSegmentFunction::as_string): Likewise.
	(ConstGenericParam::accept_vis): Likewise.
	(PathInExpression::accept_vis): Likewise.
	(TypePathSegment::accept_vis): Likewise.
	(TypePathSegmentGeneric::accept_vis): Likewise.
	(TypePathSegmentFunction::accept_vis): Likewise.
	(TypePath::accept_vis): Likewise.
	(QualifiedPathInExpression::accept_vis): Likewise.
	(QualifiedPathInType::accept_vis): Likewise.
	(GenericArg::disambiguate_to_const): Likewise.
	(GenericArg::disambiguate_to_type): Likewise.
	* ast/rust-path.cc: New file.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>