graph_files_merge() uses memory mapped files (mmap)
Memory mapping for ctx_infer_edges.c; Fix compilation issues

ctx_infer_edges.c:
- use memory mapped files when editing a file in place
- detect when reading from a stream by using stat()
- full suite of tests in tests/inferedges/Makefile

infer_edges.c:
- use either covgs or bitarray node_in_col to test if kmer in colour

Minor changes:

1. Convert comma operators to 'compound statements within expressions' (GNU C):
    e.g. ({ ctx_assert(); val; })
   This should fix compilation issues when asserts are turned off

2. Asserts+checks now turned off with `make RELEASE=1`
Test script [tests/run.sh] fetches dependencies before running