txscript: Reduce script parse opcode allocs. (#677)
txscript: Reduce script parse opcode allocs. (#677)

This changes the script template parsing function to use a pointer into
the constant global opcode array for parsed opcodes as opposed to making
a copy of the opcode entries which causes unnecessary allocations.

Profiling showed that after roughly 48 hours of operation, this
copy was the culprit of 207 million unnecessary allocations.
jrick/btcd,Stash-Crypto/btcd,btcsuite/btcd,Roasbeef/btcd,dajohi/btcd,ltcsuite/ltcd,jcmincke/dcrd,tb00/btcd,stevenroose/btcd,adiabat/btcd,hectorj/btcd,Stash-Crypto/btcd,BitGo/prova,jcmincke/dcrd,jcmincke/dcrd,cqswxj2013/btcd,adiabat/btcd,btcsuite/btcd,davecgh/btcd,soapboxsys/ombfullnode,jongillham/btcd,aakselrod/btcd,hectorj/btcd,aakselrod/btcd,davecgh/btcd,decred/dcrd,decred/dcrd,jrick/btcd,ppcsuite/ppcd,dajohi/btcd,dajohi/btcd,cqswxj2013/btcd,ltcsuite/ltcd,soapboxsys/ombfullnode,wallclockbuilder/btcd,wallclockbuilder/btcd,Roasbeef/btcd,BitGo/prova,jongillham/btcd,jrick/dcrd,ppcsuite/ppcd,jrick/dcrd,stevenroose/btcd,tb00/btcd,jrick/btcd