nim: include all supporting tools

This PR makes a few changes to how things are done:

a) build and install "koch" - the nim make-type tool
b) use "koch" to bootstrap nim
c) build additional supporting tools such as nimble, nimgrep and nimsuggest
d) nim can use other c compilers than gcc, so instead of forcing gcc we use the one from stdenv
e) run the full test suite

We do not need the "nimble" package any longer as it is part of nim.