(Fixed handling of out-of-bounds keys.)

String keyed load used to call STRING_CHAR_AT builtin that performs
two steps (get a char code, construct a one-char string from the
code), both of which have fast cases implemented as inline runtime
functions. In this chage most of the code from these functions is
extracted to a set of common generator functions in StringStubBase and
the fast cases are grouped together in the IC code.

Review URL: http://codereview.chromium.org/1582041

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4450 ce2b1a6d-e550-0410-aec6-3dcde31c8c00