py: Take gc_pool_start out of bss section, to reclaim 1st block of heap.
py: Fix compiling of nested while/for and exception handler.

Addresses issue #912.
py: Add more debug printing code in gc_dump_alloc_table.
tests: Add test for nested while with exc and break.
py: Simplify compilation of elif blocks.
py: Add more compiler optimisations for constant if/while conditions.
modure: Update to re1.5 v0.6.1, fixed and extended character class support.