This also removes nixcrpkgs and OSX cross compilation in favour of
compiling on the target. x86_64/musl targets are still supported
on Linux.
All sources are now managed via niv (see nix/sources.json) and Haskell
package sets are provided/organised via IOHK's haskell.nix.
Some effort has been made to expose similar top-level attributes for
development, but in some cases there have been changes. Please see
the comments in the top-level default.nix and ci.nix files for usage.
0bdced981e introduced the 'arvo-ropsten' derivation. Attempting to
install both 'arvo' and 'arvo-ropsten' via nix-env will result in a
priority error; this assigns a higher priority to 'arvo' to resolve the
conflict.
Fixes#1912.