Much cleaner strings with index_extensionality (and for Seq and ImmutableArray) that does not create a new eq.

[briangmilnes]

Here is a much cleaner FStar.Strings.* which proves index_extensionality instead of defining a new eq.
Once I had it there, I also proved it for Seq and ImmutableArray.

The biggest question that I have, due to FStar.String.fsti being implemented only in FStar_String.ml I had to make
this FStar.String.{Base,Properties} which must be separately included ONLY for the new lemmas and functions.

FStar.String.Match contains first_diff as we are sure to end up adding some string matching someday.

There are two test files, one validation time, Test.FStar.String and one runtime that mere prints Test.FStar.String.TestMain
as with interfaces some functionality can not be tested at assert time. When Final is ready we might move the runtime tests
to that, once it is accepted.

[briangmilnes]

Interestingly this checkin left around some file references in my next branch (from master) that appeared in test. What's the right way to clean up test? make clean; make -j20; make -j20 ci did not do the job.