StateStore: added noexcept specifier to special member functions.
StateStore: added get_states(), set_masked(), and remove().

test/utils/statestore: added tests for new functions.
Replaced use of assert() with DUCT_ASSERTE(); tidy.

IO::dynamic_streambuf,
IO::memstream,
Variable,
test/io,
test/text,
test/utils,
test/var:
	replaced use of assert() with DUCT_ASSERTE().

test/util/endian: corrected output strings for ref swap; tidy.
StateStore: corrected GCC "defect" by explicitly default-initializing m_value.¹

1: Strange that Clang doesn't warn about this. I guess their heuristics aren't
up to snuff. (Neither GCC nor Clang zero-initialize aggregates in a defaulted
default ctor. Good to keep in mind.)
IO::dynamic_streambuf: avoid shoddy -Wreturn-type heuristics in GCC 4.7.3 with hack.
README: added Building and Defects sections; wrapped to 80 columns.

doc/pages/index: updated to README.