memory_io: renamed MemoryStream::get_buffer() to buffer(); tidy.
memory_io: added MemoryReader class.
kvs: added back().
kvs: added missing is_X() type checks; tidy.
memory_io: corrected status for incomplete read.¹

1: Nothing is failing, so it's only EOF (partial read
when read_size != nullptr).
kvs: added read() and write() (IO interface).
test/kvs/values: added collection tests.
test/io/memory: added MemoryReader test.
test/kvs/io: new test.
kvs: corrected IO formats; added __format__ attribute to writef().
math: added GCC push/pop for -Wfloat-equal.¹

1: Don't have any float comparison utilities yet, so this
is flying for the tests for now.
kvs: corrected vector output in kvs::write().
kvs: added vector parsing to read().
test/kvs/io: added vector tests.