lib: json: Efficiently pack field name, offset, alignment, type

This trades a little bit over 40 bytes (on x86) of text for a lot of
savings in rodata.  This is accomplished by using bitfields to pack the
field name length, offset, alignment, and the type tag into a single
32-bit unsigned integer instead of scattering this information into
four different integers.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>