improve assertions message on function pointer errors
mention ASSERTIONS in abort() calls
extra testing for ASSERTIONS == 2
add test for #2175
fix other.test_dangerous_func_cast
sort function signatures in ASSERTIONS=2 abort message dumps, attempting to show most likely related ones first
add test for phi with unreachable source with a 64-bit value
mention that EMSCRIPTEN_KEEPALIVE also exports for you
Merge branch 'incoming' of https://github.com/kripken/emscripten into incoming