Add String#prepend
Merge pull request #2674 from suzukaze/string.prepend

Add String#prepend
mrb_str_new(mrb, "", len) creates an unmodifiable string object; ref #2674
avoid out-of-bounds access of irep->lv; ref #2671, fix #2675

allocation size of irep->iv is irep->nlocals-1.
mruby-eval: proc from env may be NULL; close #2676
String#[] should reject nil index
try to convert not only nil but every objects to fixnums; ref #2677
Merge branch 'h2so5-string-nil-index'
String#[]: float handling merged to #2677