abg-writer: Refactor write_corpus_group API

abg-writer: Refactor write_corpus_group API

Introduce a new call overload for write_corpus_group that follows the
parameter order context, object (i.e. corpus_group), indent.

Deprecate all other overloads that were part of the API and mostly
forward them to the new overload. 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_corpus_group): Introduce new
	overload write_corpus_group(ctxt, corpus_group, indent) and
	deprecate all others.
	* src/abg-writer.cc (write_corpus_group): Likewise for the
	definitions and adjust.
	* tools/abidw.cc (load_kernel_corpus_group_and_write_abixml):
	Migrate to new API of write_corpus_group()

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