Adding path-relative-to utility, plus pathname cleanup and tests.
(fork) now kills other threads atommically in the child.  Use
(%fork) to keep other threads running in both parent and child.
Adding setenv/unsetenv.
Adding procedure-arity and procedure-variadic?.
Adding path-strip-leading-parents.
Adding create-directory*.
Adding memoization library.
Patch for better error reporting in numeric tests from Per Bothner.
Updating compose example to force exact result.
Adding set-current-environment! and using it in the pure-Scheme load.
This is necessary if the loaded file uses import.