Optimize readTxIn.
Optimize readTxIn.

Before: BenchmarkReadTxIn        1000000              2435 ns/op
After:  BenchmarkReadTxIn        1000000              1427 ns/op

This is part of the ongoing effort to optimize serialization as noted in
conformal/btcd#27.

Stash-Crypto/btcd,danoctavian/btcd,toddfries/btcd,Dirbaio/btcd,ppcsuite/ppcd-old,ppcsuite/ppcd,Dirbaio/btcd,Roasbeef/btcd,btcsuite/btcd,runeaune/btcd,qchain/btcd,jcmincke/dcrd,BitGo/prova,jongillham/btcd,jongillham/btcd,decred/dcrd,davecgh/btcd,cjepson/btcd,qchain/btcd,tb00/btcd,ltcsuite/ltcd,aakselrod/btcd,Roasbeef/btcd,runeaune/btcd,ppcsuite/ppcd,pkothbauer/btcd,btcsuitereleases/btcd,btcsuitereleases/btcd,toddfries/btcd,adiabat/btcd,stevenroose/btcd,cjepson/btcd,DanielKrawisz/btcd,1d4Nf6/btcd,Cleawing/btcd,adiabat/btcd,Cleawing/btcd,jrick/btcd,dan-da/btcd,Stash-Crypto/btcd,jimmysong/btcd,wallclockbuilder/btcd,wallclockbuilder/btcd,hectorj/btcd,ltcsuite/ltcd,glv2/ppcd,Dirbaio/btcd,jimmysong/btcd,ppcsuite/ppcd-old,btcsuite/btcd,dajohi/btcd,dajohi/btcd,pkothbauer/btcd,tb00/btcd,jcmincke/dcrd,glv2/ppcd,dan-da/btcd,mathiasrw/btcd,aakselrod/btcd,FactomProject/btcd,jrick/dcrd,dajohi/btcd,davecgh/btcd,stevenroose/btcd,cqswxj2013/btcd,mathiasrw/btcd,BitGo/prova,DanielKrawisz/btcd,soapboxsys/ombfullnode,soapboxsys/ombfullnode,1d4Nf6/btcd,danoctavian/btcd,CryptocurrencyCabal/htcd,jrick/btcd,jrick/dcrd,decred/dcrd,FactomProject/btcd,CryptocurrencyCabal/htcd,jcmincke/dcrd,cqswxj2013/btcd,jrick/btcd,hectorj/btcd