This adds initial support for on-demand inputs by allowing new inputs to
be created with only a shared reference to the database. This allows
creating new inputs during a revision and therefore from inside tracked
functions.
369: Try to fix issue#340 r=XFFXFF a=zjp-CN
(WIP)
Follow the instructions from https://github.com/salsa-rs/salsa/issues/340#issuecomment-1210153085.
Still not sure what to do with `fmt_index`...
Fix#340
Co-authored-by: zjp <jiping_zhou@foxmail.com>
Co-authored-by: Niko Matsakis <niko@alum.mit.edu>