Cabal support

Asterius now has preliminary Cabal support. By substituting toolchain executables like ghc/ghc-pkg and supplying some other configure options, Cabal can build static libraries and "executables" using Asterius. The "executables" can be quickly converted to node/web artifacts using ahc-dist.

We also provide ahc-cabal which is a wrapper for cabal. ahc-cabal works with typical nix-style commands like new-update/new-build, etc. The legacy commands with v1 prefix may also work.