Rollback using cgen/seq for values because the generated substitutions have issues.
add `unix-style` makefile target

Also, revise "INSTALL.txt" to better explain the build options.
add scheme_jit_now()
clean up initialization of the teachpacks in an empty repl

In more detail, the way the initial namespace was being set up was
different than the way that you get if you just hit 'Run' on an
empty program. So change the initial namespace creation to more closely
mimic the way an running an empty program works.
fix timeout computation in the REPL io

In the case that the flush queue was empty, the first byte that
comes it would appear to have come from a long time ago and
thus get flushed immediately.

Instead, when the first byte comes in, we restart the timer at that
point, intead of holding the old value
do a better job of keeping the err/out ports IO in the right order

... in the case that IO happens during the 'first-opened' method,
e.g., during teachpack running in an empty REPL
Re-enable Values and cgen/seq, and fix contracts on substitute.

After longer testing confirmed that this was just to strict contracts on
substitute.
Unset contracts enabled by default for TR.
Make subst-gen not duplicate call to values.
Remove unused code in subst-gen, and remove internal error.
Simplify constraint->type.
Remove unused variable in constraint struct.
Make promote-demote no longer a unit.
Add tests for promote/demote.

Also fix promotion/demotion in hashtables and kw arguments.
Merge var-promote and var-demote.
Make initial version of structural type recursion, and use it.
Add performance hint to var-promote.