* m4/labels-as-values.m4: New file, checks for computed goto.
* configure.in: Use AC_C_LABELS_AS_VALUES.
* module/system/repl/command.scm (procedure-documentation): Extend the
core's procedure-documentation in an ad-hoc way, so that ,help works.
* module/system/vm/core.scm (program-properties): New function.
(program-documentation): New function.
* src/vm_engine.h (DROP, DROPN): Decrement sp before checking for
underflow.
* src/vm_system.c (call, tail-call): Add some optimized dispatch for some
C functions, so that we can avoid consing and the interpreter if
possible. However currently it seems that I'm always getting the
scm_call_* trampolines back.
