If the user types something like `/**`, prior to this change we would
attempt to expand it in the background for both highlighting and
autosuggestions. This could thrash your disk and also consume a lot of
memory.
Add a a field to operation_context_t to allow specifying a limit, and add
a "default background" limit of 512 items.
This commit recognizes an existing pattern: many operations need some
combination of a set of variables, a way to detect cancellation, and
sometimes a parser. For example, tab completion needs a parser to execute
custom completions, the variable set, should cancel on SIGINT. Background
autosuggestions don't need a parser, but they do need the variables and
should cancel if the user types something new. Etc.
This introduces a new triple operation_context_t that wraps these concepts
up. This simplifies many method signatures and argument passing.
