We already did this, but missed a few cases due to the way arguments are
elaborated. So now, when checking an LHS, we don't allow LHS argument
types to be inferred from the pattern, but rather they must be inferred
from elsewhere. To do this, we keep track of the constraints which would
be solved when inferring the type, and make sure they don't solve any
new metavariables. Fixes#1510, and also now gets the error location
right as a bonus!
I'm playing with some linear structures and finding these useful a lot,
so good to have a consistent syntax for it. '#' is chosen because it's
short, looks a bit like a cross if you look at it from the right angle
(!) and so as not to clash with '@@' in preorder reasoning syntax.
Can't export a type which refers to a private name. This has caught a
couple of visibility errors in the libraries, code and tests, so they've
been updated too.
