Implement better diagnostics

[darkf]

Currently if some mundane error happens, such as a pattern failure, you get a cryptic and unlocalized error such as argument 1 doesn't satisfy any patterns.

Ideally these errors would contain with them the module and line number the error occurred in. This will require tracking the line number associated with AST nodes.

It might also be a good idea to pretty-print functions, so that we can do:

argument 1 doesn't satisfy any patterns in the function `f() -> 1.`

If the function body is too long, it will be truncated, such as:

argument 1 doesn't satisfy any patterns in the function `f() -> if true then if true then ... else (,) else (,).`

or:

argument 1 doesn't satisfy any patterns in the function `f() -> x + y + z ....`