- add unified frontend configuration record to make adding future
  options easier
- use ExceptT throughout, rather than using runExceptT at internal
  boundaries

In places where an optional space was previously allowed, allow any
number of arbitrary whitespace characters, in line with the spec and
other tools.

- number literals are parsed rather than stored as strings
- fix array query functions used on non-trivial number literals
- more efficient expression simplification recursion
- expanded constant folding scenarios

This reverts commit ba7fbd55.

This is the same as most compilers (e.g. gcc and clang) but also
verilog tools (e.g. yosys) output error mesasge in a standardized form.

This form can immediately be parsed by IDE tools that can jumping
through a list of error messages, placing the cursor at the given
file position.

This change simply re-arranges the error message printing to conform to that
standard.

Signed-off-by: Henner Zeller <h.zeller@acm.org>