Modify the result string in place when using to_sub_replacement
Modify the result string in place when using to_sub_replacement

This greatly improves performance if a lot of small parts
are replaced in a string using sub / gsub. Before to_sub_replacement
allocated a string object in each iteration, which could cause
a lot of GC pressure in large strings with a lot of replacements.

The performance of the benchmark added for single character replacements
in the previous comment increases by 50% with this change.

jsyeo/rubinius,dblock/rubinius,heftig/rubinius,jsyeo/rubinius,mlarraz/rubinius,Wirachmat/rubinius,slawosz/rubinius,pH14/rubinius,jemc/rubinius,dblock/rubinius,travis-repos/rubinius,pH14/rubinius,heftig/rubinius,kachick/rubinius,mlarraz/rubinius,lgierth/rubinius,jemc/rubinius,ruipserra/rubinius,sferik/rubinius,slawosz/rubinius,Wirachmat/rubinius,benlovell/rubinius,kachick/rubinius,benlovell/rubinius,slawosz/rubinius,heftig/rubinius,kachick/rubinius,jemc/rubinius,pH14/rubinius,kachick/rubinius,pH14/rubinius,digitalextremist/rubinius,digitalextremist/rubinius,benlovell/rubinius,slawosz/rubinius,jsyeo/rubinius,benlovell/rubinius,ruipserra/rubinius,jsyeo/rubinius,benlovell/rubinius,lgierth/rubinius,Azizou/rubinius,mlarraz/rubinius,digitalextremist/rubinius,ngpestelos/rubinius,Wirachmat/rubinius,lgierth/rubinius,ngpestelos/rubinius,heftig/rubinius,ruipserra/rubinius,sferik/rubinius,jsyeo/rubinius,Wirachmat/rubinius,pH14/rubinius,pH14/rubinius,sferik/rubinius,travis-repos/rubinius,pH14/rubinius,Azizou/rubinius,travis-repos/rubinius,ngpestelos/rubinius,ruipserra/rubinius,heftig/rubinius,dblock/rubinius,digitalextremist/rubinius,kachick/rubinius,slawosz/rubinius,sferik/rubinius,jemc/rubinius,heftig/rubinius,travis-repos/rubinius,ruipserra/rubinius,lgierth/rubinius,kachick/rubinius,Azizou/rubinius,ruipserra/rubinius,sferik/rubinius,digitalextremist/rubinius,jsyeo/rubinius,travis-repos/rubinius,Azizou/rubinius,ngpestelos/rubinius,benlovell/rubinius,digitalextremist/rubinius,sferik/rubinius,mlarraz/rubinius,Wirachmat/rubinius,mlarraz/rubinius,Azizou/rubinius,jemc/rubinius,jemc/rubinius,dblock/rubinius,jemc/rubinius,benlovell/rubinius,kachick/rubinius,jsyeo/rubinius,mlarraz/rubinius,kachick/rubinius,lgierth/rubinius,heftig/rubinius,mlarraz/rubinius,Wirachmat/rubinius,lgierth/rubinius,dblock/rubinius,ngpestelos/rubinius,slawosz/rubinius,ruipserra/rubinius,lgierth/rubinius,sferik/rubinius,ngpestelos/rubinius,digitalextremist/rubinius,travis-repos/rubinius,Wirachmat/rubinius,ngpestelos/rubinius,dblock/rubinius,slawosz/rubinius,dblock/rubinius,Azizou/rubinius,Azizou/rubinius,travis-repos/rubinius