Style and doc tidy.
TieBinder: made compound ctor explicit.  Style tidy.
Implemented immutable tie binding; see full commit log.

Tie: added is_mutable().

TieBinder:
	added const-object bind;
	made primitive bind operator()s inline.

TieCompound:
	made bind_begin() require explicit bind mode;
	made bind_object() choose bind mode based on object constness;
	moved state management functions into class;
	added is_mutable().

detail::primitive_type_to_desc: added is_const.

serialize: added sanity-check assertions to deserialize().

test/Generic: added immutable bind().
test/nested_comp: added immutable bind() for classes A, B, C, and D.
test/desc/serialize: made out const.