Add a select_const for selecting the const method implementation if there are only two.

Conflicts:
	tests/embind/embind_test.cpp
snip dead code
namespace the binding struct a bit more
Snip some dead code
checkpoint object handle composition
Add isAliasOf for seeing if two embind handles point to the same underlying object.

Conflicts:
	tests/embind/embind_test.cpp
Add a benchmark for calling from C++ into JS via interfaces

Conflicts:
	emscripten.py
	tests/embind/embind.benchmark.js
Add a test for interface methods that return void.
Oops, fix a test leak.