gccrs: Move templated functions to header file

Templated functions shall remain in header files to stay in line with the
rest of the codebase.

gcc/rust/ChangeLog:

	* ast/rust-ast-collector.cc (TokenCollector::visit): Move to header
	file.
	(TokenCollector::visit_items_joined_by_separator): Likewise.
	(TokenCollector::visit_as_line): Likewise.
	(TokenCollector::visit_items_as_lines): Likewise.
	(TokenCollector::visit_items_as_block): Likewise.
	* ast/rust-ast-collector.h: Add implementation.

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