runtime: optimize slicebytestostring
runtime: optimize slicebytestostring

Inline rawstringtmp and simplify.
Use memmove instead of copy.

name                     old time/op  new time/op  delta
SliceByteToString/1-8    19.4ns ± 2%  14.1ns ± 1%  -27.04%  (p=0.000 n=20+17)
SliceByteToString/2-8    20.8ns ± 2%  15.5ns ± 2%  -25.46%  (p=0.000 n=20+20)
SliceByteToString/4-8    20.7ns ± 1%  14.9ns ± 1%  -28.30%  (p=0.000 n=20+20)
SliceByteToString/8-8    23.2ns ± 1%  17.1ns ± 1%  -26.22%  (p=0.000 n=19+19)
SliceByteToString/16-8   29.4ns ± 1%  23.6ns ± 1%  -19.76%  (p=0.000 n=17+20)
SliceByteToString/32-8   31.4ns ± 1%  26.0ns ± 1%  -17.11%  (p=0.000 n=16+19)
SliceByteToString/64-8   36.1ns ± 0%  30.0ns ± 0%  -16.96%  (p=0.000 n=16+16)
SliceByteToString/128-8  46.9ns ± 0%  38.9ns ± 0%  -17.15%  (p=0.000 n=17+19)

Change-Id: I422e688830e4a9bd21897d1f74964625b735f436
Reviewed-on: https://go-review.googlesource.com/37791
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Marvin Stenger <bde3f4ccdd7c08fc2bda30f654959132eeced9b3@gmail.com>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

polyverse-security/go,derekmarcotte/go,polyverse-security/go,ImJasonH/go,linux-on-ibm-z/go,linux-on-ibm-z/go,codestation/go,vsekhar/elastic-go,akutz/go,nawawi/go,polyverse-security/go,vsekhar/elastic-go,CAFxX/go,golang/go,akutz/go,golang/go,derekmarcotte/go,vsekhar/elastic-go,linux-on-ibm-z/go,akutz/go,CAFxX/go,derekmarcotte/go,akutz/go,codestation/go,Cofyc/go,cloudflare/go,codestation/go,akutz/go,nawawi/go,vsekhar/elastic-go,cloudflare/go,akutz/go,codestation/go,ImJasonH/go,polyverse-security/go,josharian/go,Cofyc/go,derekmarcotte/go,polyverse-security/go,ImJasonH/go,vsekhar/elastic-go,vsekhar/elastic-go,codestation/go,nawawi/go,nawawi/go,linux-on-ibm-z/go,polyverse-security/go,CAFxX/go,josharian/go,akutz/go,golang/go,nawawi/go,codestation/go,CAFxX/go,vsekhar/elastic-go,josharian/go,josharian/go,derekmarcotte/go,cloudflare/go,derekmarcotte/go,Cofyc/go,Cofyc/go,josharian/go,nawawi/go,golang/go,akutz/go,nawawi/go,ImJasonH/go,CAFxX/go,golang/go,codestation/go,cloudflare/go,vsekhar/elastic-go,josharian/go,linux-on-ibm-z/go,golang/go,Cofyc/go,nawawi/go,linux-on-ibm-z/go,cloudflare/go,cloudflare/go,josharian/go,derekmarcotte/go,linux-on-ibm-z/go,golang/go,polyverse-security/go,cloudflare/go,golang/go,ImJasonH/go,Cofyc/go,CAFxX/go,ImJasonH/go,CAFxX/go,polyverse-security/go,Cofyc/go,Cofyc/go,josharian/go,CAFxX/go,ImJasonH/go,codestation/go,christopher-henderson/Go,ImJasonH/go,cloudflare/go,linux-on-ibm-z/go,derekmarcotte/go