Strip bytecode for particular graph

Closes #56 (closed)

To do:

• Prevent generating dead code by overwriting labels (see TODO in strip.c)
• Add start field to bytecode header; in linker add option to select the main module
• Add bcstrip binary to strip binary leaving only the Start rule and all code reachable from there
• Use bcstrip in run_tests.sh for CI

Here is how much it saves on the tests (from least to most):

test	old size	new size	relative size
weird_types.bc	24300	13834	56.93%
fills.bc	20883	10761	51.53%
prolog.bc	213379	36861	17.27%
arrays.bc	772635	130520	16.89%
parser.bc	210728	33733	16.01%
long_integers.bc	374823	54807	14.62%
reals.bc	123786	16885	13.64%
mulmat.bc	113698	15473	13.61%
chars.bc	111320	13903	12.49%
symsieve.bc	112226	13802	12.30%
war_seq.bc	111040	13116	11.81%
functions.bc	110782	12664	11.43%
invperm.bc	109769	11635	10.60%
twice.bc	108805	10811	9.94%
revtwice.bc	119637	11436	9.56%
stwice.bc	108808	10383	9.54%
str_arit.bc	113172	10717	9.47%
fsieve.bc	121779	11389	9.35%
acker.bc	108542	10119	9.32%
tak.bc	108529	10106	9.31%
nfib.bc	108468	10045	9.26%
iks.bc	786870	71059	9.03%
cafs.bc	187591	16162	8.62%
rfib.bc	120669	10077	8.35%
pascal.bc	189435	14998	7.92%
compress.bc	709375	56150	7.92%
e.bc	185755	13835	7.45%
infprimes.bc	184853	13351	7.22%
revrec.bc	189347	13255	7.00%
sieve.bc	184526	12807	6.94%
hamming.bc	184665	12729	6.89%
match.bc	184783	12565	6.80%
lqueen.bc	185430	12491	6.74%
brainfuck.bc	318020	21366	6.72%
reverse2.bc	184939	11994	6.49%
reverse.bc	184047	11912	6.47%
squeen.bc	184668	11527	6.24%
matchlarge.bc	203511	12695	6.24%
reversearray.bc	185622	11048	5.95%
sudoku.bc	655488	29313	4.47%
unboxed_arrays.bc	311144	10698	3.44%