While the backends will usually manage to find the constant size anyway,
this ensures that implicit arguments will be filled in with the constant
value too.  (For example, this was affecting isla execution in one corner
case because the slice_mask primitive didn't see that the size was
constant.)

Also fix a few shellcheck warnings related to printf while doing so.

Both /.gitignore and /lib/coq/.gitignore ignored some files in /lib/coq.
This commit removes /lib/coq/.gitignore and moves all ignore-statements
to /.gitignore . This should simplify the maintenance of gitignore files.

The situation with /test/mono/.gitignore is analogous.

- add tests for a couple of related rewrites
- accept same range of constants for sign extension in the rewrite as for
  the zero extension version (to make the test simpler)

The monomorphisation analysis decides whether to split function
arguments in the callee or in callers.  The code previously used a
datastructure that can hold results of either the one case or the other,
but there might be functions that are called in different contexts
leading to different decisions.  This patch changes the datastructure to
support storing all instances of either case.

If we call a function where some arguments need to be rewritten, we
might need to rewrite those parameters in the caller as well.

For example, in

  let datasize = e in ...

the typechecker will generate a kid '_datasize if e has an existential
type (with one kid), and in

  let 'datasize = e in ...

the typechecker will bind both 'datasize and '_datasize.  If we
substitute one as part of constant propagation, this patch will make
constant propagation also substitute the other.

... and try to resolve them using constant propagation.