Use stacked context for nested bindings.

Use stacked context for nested bindings.

Binding context may be stacked when a new binding group is introduced
within a const expression.

gcc/rust/ChangeLog:

	* resolve/rust-name-resolution-context.h: Use BindingLayer instead.
	* resolve/rust-name-resolution-context.cc (BindingLayer::BindingLayer):
	Add new constructor for binding layer.
	(BindingLayer::bind_test): Add a function to test a binding constraint.
	(BindingLayer::push): Push a new binding group.
	(BindingLayer::and_binded): Add function to test and-binding
	constraint.
	(BindingLayer::or_binded): Add function to test or-binding constraints.
	(BindingLayer::insert_ident): Insert a new identifier in the current
	binding group.
	(BindingLayer::merge): Merge current binding group with it's parent.
	(BindingLayer::get_source): Get the source of the current binding
	group.
	* resolve/rust-late-name-resolver-2.0.cc: Use stacked context for
	binding group.
	* util/rust-stacked-contexts.h: Add mutable peek function.

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