build script: rebuild on changes to *.[ch] too
plugins: automatically import fubsy module into Python's __main__
build scripts: print required environment variables more clearly and consistently
runtime: add BuiltinList, a new Namespace implementation for builtins

It preserves order, which will be useful for calling builtins from C
(e.g. embedded Python, Lua) -- we'll just look them up in a table,
which is easy in C. It also disallows assignment by panic()ing.
plugins: handle callbacks to builtins more dynamically

Instead of a hardcoded set of void pointers to Go callbacks, use an
array of void pointers. It's still hardcoded to the current set of
builtins, but it's progress!
runtime: add support for evaluating list literals
runtime: generate C wrappers for Python to call builtins

Since it's impossible to generate C functions at runtime, we need to
generate them at build time.
remove local copy of gocov -- download + build it in configure.sh instead
testutils: add NoError(), and make NoErrors() fail fatally if it fails
plugins: remove some debugging prints
plugins: more tests; use new testutils.NoError() function