Add support for signed 24.8 decimal numbers

Add support for signed 24.8 decimal numbers

'fixed' is a signed decimal type which offers a sign bit, 23 bits of
integer precision, and 8 bits of decimal precision.  This is exposed as
an opaque struct with conversion helpers to and from double and int on
the C API side.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>