Add inline version of String.indexOf().

Add inline version of String.indexOf().

This provides an inline-native version of String.indexOf(int) and
String.indexOf(int, int), i.e. the functions that work like strchr().
Has a fairly solid impact on specific benchmarks.  Might give a boost to
an app somewhere.

Added some indexOf tests to 020-string.

Added hard-coded field offsets for String.  These are verified during
startup.  Improves some of our String micro-benchmarks by ~10%.