New field descriptor format.

Now field information is stored as a variable-length uint32_t array.
This reduces the minimum size in most cases:

                            old            new
basic field:                10 bytes       4 bytes
repeated field:             10 bytes       8 bytes
field with 12bit tag/size:  16 bytes       8 bytes
field with 16bit tag/size:  16 bytes       16 bytes
field with 32bit tag/size:  28 bytes       16 bytes

Further, now the descriptor size can be increased per-field and per-message,
instead of previous global PB_FIELD_16BIT and PB_FIELD_32BIT.

PB_FIELD_32BIT still affects the pb_size_t type, which is 16-bit by default.