Fix exception/error state when using static_buffer with buffer_adaptor

[Chaosvex]

static_buffer wasn't previously usable with buffer_adaptor but the changes made to allow for writable std::arrays have made it compatible.

Due to the interface differences, using static_buffer with buffer_adaptor would throw an exception (or set the error state) since size() reports the number of readable bytes available, unlike std::array, where it's a capacity constant. This caused writing to static_buffer to fail with an out of space error.

Adding resize() allows buffer_adaptor to effectively advance the write cursor and use it as it would any other contiguous buffer.

There's no reason to use static_buffer with buffer_adaptor, but may as well fix it so it works.