remove warning about missing 'const' support from docs and fix advice on declaring macros as 'enum' (which works only for ints)

Fix overflow_check CompileError with msvc

--HG--
extra : transplant_source : %3FH%B6t%1FFL%D8%2BF%7CV%7D%10T9%EA%CB%A0%00

enable control flow analysis for stack allocated structured variables (struct, union, array, C++ class)

rewrite except-as semantics using try-finally - seems to be the easiest way to get flow control analysis working for all cases

Add test for memoryview of extension type