libtrellis: offer serialise_chip/serialise_chip_delta in Python api

[rrika]

This commit makes two functions to serialize full and partial bitstreams usable from python. The C++ signatures are

static Bitstream serialise_chip(const Chip &chip, const map<string, string> options);
static Bitstream serialise_chip_partial(const Chip &chip, const vector<uint32_t> &frames, const map<string, string> options);

The issues with wrapping these are:

1. I can't figure out how to make boost-python allow passing the maps/vectors for the arguments.
2. serialise_chip_partial takes a list of frames, which AFAICT can currently only be built from python by calling get_bit on both chips for every bit, or looping over a CRAMDelta (which occupies 12 bytes per changed bit)

This patch instead adds

static Bitstream serialise_chip_py(const Chip &chip);
static Bitstream serialise_chip_delta_py(const Chip &chip1, const Chip &chip2);

Usage example:

#!/usr/bin/env python3

import sys; sys.path.append("[...]/prjtrellis/libtrellis")
import pytrellis; pytrellis.load_database("[...]/prjtrellis/database")

chip1 = pytrellis.Bitstream.read_bit("before.bit").deserialise_chip()
chip2 = pytrellis.Bitstream.read_bit("after.bit").deserialise_chip()
bit_delta = pytrellis.Bitstream.serialise_chip_delta(chip1, chip2)
bit_delta.write_bit("delta.bit")