Improve performance of String#*
Improve performance of String#*

This changes two things. First it alloces a dirty byte array so we don't
zero the byte array storage and immediately overwrite it.

Seconds is a smarter strategy for creating the pattern by using doubling
as long as we can and then copy the final data in a single go.

Before:
=== bin/rbx ===
            String#*    58992.9 (±1.9%) i/s -     297055 in   5.037281s

After:
=== bin/rbx ===
            String#*   635072.8 (±12.3%) i/s -    3131848 in   5.015714s

pH14/rubinius,jsyeo/rubinius,Wirachmat/rubinius,heftig/rubinius,sferik/rubinius,jemc/rubinius,ngpestelos/rubinius,ruipserra/rubinius,jsyeo/rubinius,mlarraz/rubinius,heftig/rubinius,ngpestelos/rubinius,kachick/rubinius,sferik/rubinius,sferik/rubinius,pH14/rubinius,pH14/rubinius,ruipserra/rubinius,benlovell/rubinius,ngpestelos/rubinius,kachick/rubinius,Azizou/rubinius,digitalextremist/rubinius,heftig/rubinius,pH14/rubinius,mlarraz/rubinius,Wirachmat/rubinius,dblock/rubinius,Azizou/rubinius,benlovell/rubinius,mlarraz/rubinius,benlovell/rubinius,ngpestelos/rubinius,ruipserra/rubinius,benlovell/rubinius,Wirachmat/rubinius,jemc/rubinius,digitalextremist/rubinius,sferik/rubinius,digitalextremist/rubinius,benlovell/rubinius,Azizou/rubinius,Wirachmat/rubinius,kachick/rubinius,ngpestelos/rubinius,sferik/rubinius,benlovell/rubinius,ruipserra/rubinius,jsyeo/rubinius,pH14/rubinius,kachick/rubinius,ngpestelos/rubinius,ruipserra/rubinius,digitalextremist/rubinius,kachick/rubinius,benlovell/rubinius,pH14/rubinius,pH14/rubinius,mlarraz/rubinius,dblock/rubinius,digitalextremist/rubinius,dblock/rubinius,digitalextremist/rubinius,sferik/rubinius,kachick/rubinius,ruipserra/rubinius,Azizou/rubinius,jemc/rubinius,Azizou/rubinius,dblock/rubinius,jemc/rubinius,Wirachmat/rubinius,heftig/rubinius,jsyeo/rubinius,kachick/rubinius,jsyeo/rubinius,Azizou/rubinius,Wirachmat/rubinius,mlarraz/rubinius,jemc/rubinius,ruipserra/rubinius,ngpestelos/rubinius,dblock/rubinius,mlarraz/rubinius,Azizou/rubinius,jemc/rubinius,kachick/rubinius,dblock/rubinius,sferik/rubinius,heftig/rubinius,digitalextremist/rubinius,Wirachmat/rubinius,dblock/rubinius,mlarraz/rubinius,heftig/rubinius,jsyeo/rubinius,jemc/rubinius,jsyeo/rubinius,heftig/rubinius