bitcoin-utils: fix encoding of CompactSize integers
Encoding an atom as a Compact fails when a = 0: a 4-byte CompactSize is
emitted.
|= a=@
^- hexb
=/ l=@ (met 3 a)
+ ?: =(l 0) 1^a
?: =(l 1) 1^a
?: =(l 2) (cat:byt ~[1^0xfd (flip:byt 2^a)])
?: (lte l 4) (cat:byt ~[1^0xfe (flip:byt 4^a)])
