Add option to dump CompileExpr in a standard, parseable format

[evertedsphere]

This seems fairly simple, and I intend to add support for this myself. The easiest way, if not the "correct" one, is to add a dummy backend that pretty-prints the CompileExpr it receives to a file.

[chrrasmussen]

I think --dumpcases is what you are looking for. It was added in commit 5950371.

If you have a Main.idr with a main : IO () function, you can use --dumpcases like this:

idris2 --dumpcases foo -o bar Main.idr

• foo is the name of the file where the CompileExpr will be dumped (located in the current directory)
• bar is the name of the output executable (located in build/exec)

[evertedsphere]

I don't think so; I guess I should've mentioned that my use-case requires the output to be easily parseable in a way that the output of --dumpcases is not, so I can use it as an input for further processing.

I discussed this with @edwinb yesterday, and it seems we may want to standardise a stable s-expression-style text format for this.