Round up allocation size to avoid misalignment (issue #226)

Before this change, requesting a large initial allocation could cause the
backing store to grow to an unaligned size. Since memory inside vector_downward
is relative to the end of the buffer, this then caused all memory in the buffer
to be misaligned and also misaligns any further loads and stores. Misaligned
loads and stores are undefined behavior and don't work in environments such as
emscripten (a JavaScript to C++ compiler).