gccrs: libproc_macro: Fix Tokenstream growth

TokenStream did not copy back enough old data to the new location. This
commit also add more explicit memcpy usages in order to facilitate
change to utf-8 later.

libgrust/ChangeLog:

	* libproc_macro/ffistring.cc (FFIString::make_ffistring):
	Add explicit sizeof and utf-8 warning.
	(FFIString::clone): Add explicit sizeof and utf-8 warning.
	* libproc_macro/ident.cc (Ident::clone): Likewise.
	(Ident::make_ident): Likewise.
	* libproc_macro/tokenstream.cc (TokenStream::grow):
	Fix vector growth.
	(TokenStream__clone): Add explicit sizeof.

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