Line directives for the lexer

[ppolesiuk]

This PR introduces line directives recognized by the lexer. For instance, in

#@ 123 foo.fram
let x = ... some error ...

the error will be reported in foo.fram at line 123.

Moreover, the lexer warns when it encounters other comments starting with #@. We might use such comments for other directives in the future.

Resolves #128

[Brychlikov]

My only worry is the legibility of error messages. Currently, if there is an error with line directive pointing to a nonexistent file or a nonexistent line in an existing file, we just quietly don't show any context. I think there should at least be a warning in this case.
I don't have much experience working with code generators, so other than that, I think it's fine.