Filepaths within a Loc must now be absolute or an error is thrown when
mkLoc is called. This Loc is used when displaying errors.
This commit uses imaginary absolute file paths in the Core repl and Asm
commands in the cases (parsing a single expression for example).
Before this fix, the `core {repl, read, eval}` and `asm` commands would
crash if it encountered an error when invoked with a relative path, or
in the case of a repl when parsing a single expression.
