Task: Add negative number literals to MExpr

[br4sco]

We have decided to add negative number literals to MExpr. I.e., you should be able to include negative integers and negative floats, e.g., -1 and -1., in MExpr programs.

To do this, we would have to make updates to at least the following files:

MExpr

stdlib/mexpr/pprint.mc
stdlib/mexpr/parser.mc

Boot

src/boot/lib/pprint.ml
src/boot/lib/parser.mly
src/boot/lib/lexer.mll

It is possible that pretty printers for other backends must also be updated to handle negative numbers.

Additionally, we want to extend pattern matching with negative numbers, but this could be a separate PR/issue.