abg-writer: Refactor write_translation_unit API

abg-writer: Refactor write_translation_unit API

Introduce a new overload for write_translation_unit that follows the
parameter order context, object (i.e. translation unit), indent.

Deprecate all other overloads that were part of the API and mostly
forward them to the new one. That effort is made to ensure write_context
is always provided. write_context allows access to all options that
influence the output format.

	* include/abg-writer.h (write_translation_unit): Declare a new
	overload write_translation_unit(ctxt, tu, indent) and deprecate
	all others.
	* src/abg-writer.cc (write_translation_unit): Likewise in the
	definitions.
	(write_corpus, dump, write_translation_unit): Adjust.
	* tools/abilint.cc (main): use new write_translation_unit() API

Signed-off-by: Matthias Maennich <maennich@google.com>