Fix exceptions_to_lua_errors.
lapi: converters.hpp is required.
test_function_converters: Internal linkage for helper function.
Add raw_function to push unwrapped lua_CFunctions.

Also supports from_stack.
Use raw_function for PUSH_FUNCTION_STATIC.
Fix LUABIND_ prefix in include guards.
push: Support multiple items.
Add rawset_table and new_table.
Test and fix registry_reference.
Move ref_mode enum outside registry_reference.
Add stack_reference.
Split converters_fwd from converters.

Also trim unused constants and typedefs from converter_base, leaving only
to_type. Keep the YAGNI principle in mind, especially for private APIs!
Add converters for Lua references (untested!).
Spare converters from useless qualifiers.

That is for push const, volatile, &, && (everything) and
top level cv for from_stack and n_conversion_steps.

The special case for const& and const&& primitive types remains. Other convertes
may avoid copies by special-casing these.
Reference converters: Push nil if empty.
Test reference converters.
Line endings: CRLF crept in somehow.

The most superfluous problem.
reference: Avoid warning about && / || with ().
lua_utils: Avoid clang warnings.
create_table: Support subtables.