Added locking to C interface (using it remains on the TODO list)

examples/simple/example_decoder.c

examples/simple/example_encoder.c

lib/c/labcomm.c

lib/c/labcomm.h

lib/c/labcomm_private.h

lib/c/test/test_labcomm_basic_type_encoding.c

lib/c/test/test_labcomm_generated_encoding.c

test/relay_gen_c.py