Implement new "Quick Start" approach

This commit does three main things:

1) Makes a copy of the top-level util/setchplenv.* scripts in a
   util/quickstart/ subdirectory and modifies them to set...

     CHPL_TASKS = fifo
     CHPL_REGEXP = none
     CHPL_GMP = none

   (the final two are not strictly necessary as this is where they
   default anyway, but with other changes that I'm working on, they
   will become necessary quickly; and don't hurt in the meantime)

   It also updates the gen_release script to include this new
   util/quickstart subdirectory.


2) Updates the top-level README to:

   - have new users use these quickstart/setchplenv.* scripts

   - give instructions for switching to the "preferred" setchplenv.*
     scripts

   - point to the PERFORMANCE file (in hopes of reducing the frequency
     of people not using the --fast flag)

   (also does some minor wordsmithing)


3) Updates the release testing script to better match the README
   instructions by:

   - using the quickstart script

   - setting the environment before making (now required for
     quickstart)


Some related TODOs are to:

* rewrite the release testing script to use a bash environment rather
  than csh since that's the use case we care most about now

* extend the release testing script to test both the quickstart and
  preferred environments

* make all nightly testing use the release tarball rather than the
  repository