nir: add serialization and deserialization

nir: add serialization and deserialization

v2 (Jason Ekstrand):
 - Various whitespace cleanups
 - Add helpers for reading/writing objects
 - Rework derefs
 - [de]serialize nir_shader::num_*
 - Fix uses of blob_reserve_bytes
 - Use a bitfield struct for packing tex_instr data

v3:
 - Zero nir_variable struct on deserialization. (Jordan)
 - Allow nir_serialize.h to be included in C++. (Jordan)
 - Handle NULL info.name. (Jason)
 - Set info.name to NULL when name is NULL. (Jordan)

Acked-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>