(evaluator): running talc forms without compiling them.

  evaluator.talc   the library; its header comment is the reference
  test.talc        the test: exit 0 = every check passed, a bit per group
  scripts/build.sh builds and runs the test
  scripts/build.talc  the same, as a talc-shell script

Build and run the test with the installed toolchain:

  compiler test.talc test && ./test

or scripts/build.sh, which does the same; from an already-built shell,
scripts/build.talc does too:

  shell scripts/build.talc

The test also runs under collection stress, where a collection happens
before every Nth allocation; the loops in it are long, so use a period:

  TALC_GC_STRESS=1000 ./test

A program that uses the library names this directory when it builds:

  compiler program.talc program /home/user/system/evaluator

What the evaluator is for: a shell, and anything else that runs talc read
at run time. It is not a second implementation of the language for
programs: it has no macros, and a program is compiled.