#693 Improve changeStr() performace

[Erich]

changeStr() seems to have room for performance improvements

[Erich]

Comitted revision [r11219] to optimize (caseless)changeStr() functions if a count arg is given

Committed revision [r11284]:

• MutableBuffer (caseless)changeStr() methods: remove duplicate scan for matches for the same-length and shorter-length case
• primitive pos() method: add check to cut down number of necessary memcmp() calls
• add MutableBuffer tests

[Erich]

Committed revision [r11285] with second MutableBuffer test