assembler: Add a regions visualizer tool

This adds a html/js tool for helping to visualise GEN instruction
source operand regions by generating diagrams similar to those used in
the Programmer's Reference Manual. To use the tool, open the file
assembler/gen_regions/gen_regions.html in your browser of preference.

Signed-off-by: Chris Cummins <christopher.e.cummins@intel.com>