[flang] Ensure that the parse tree visitation Walk template works on FORMAT.

[flang] Ensure that the parse tree visitation Walk template works on FORMAT.
The classes that were segregated into format-specification.h have
also been grouped into a new Fortran::format namespace.
Code added to tools/f18/f18.cc to run a minimal visitor over
the parse trees resulting from successful parses so that any
future build problems with Walk() will be caught earlier.

Rearrange Walk instances, keep format:: ones together.

Original-commit: flang-compiler/f18@685ddb91ceb2217877a887eca14a63a4b725ea12
Reviewed-on: https://github.com/flang-compiler/f18/pull/22