sorcery: Add ast_sorcery_retrieve_by_prefix()

Some consumers of the sorcery API use ast_sorcery_retrieve_by_regex
only so that they can anchor the potential match as a prefix and not
because they truly need regular expressions.

Rather than using regular expressions for simple prefix lookups, add
a new operation - ast_sorcery_retrieve_by_prefix - that does them.

Change-Id: I56f4e20ba1154bd52281f995c27a429a854f6a79