Add importNative primop

This can be used to import a dynamic shared object and return an
arbitrary value, including new primops. This can be used both to test
new primops without having to recompile nix every time, and to build
specialized primops that probably don't belong upstream (e.g. a function
that calls out to gpg to decrypt a nixops secret as-needed).

The imported function should initialize the Value & as needed. A single
import can define multiple values by creating an attrset or list, of
course.

An example initialization function might look like:

extern "C" void initialize(nix::EvalState & state, nix::Value & v)
{
    v.type = nix::tPrimOp;
    v.primOp = NEW nix::PrimOp(myFun, 1, state.symbols.create("myFun"));
}

Then `builtins.importNative ./example.so "initialize"` will evaluate to
the primop defined in the myFun function.
Only add the importNative primop if the allow-arbitrary-code-during-evaluation option is true (default false)
Merge branch 'shlevy-import-native'
allow-arbitrary-code-during-evaluation -> allow-unsafe-native-code-during-evaluation
Add `--json` argument to `nix-instantiate`
Style fix
Manual: html -> xhtml
Add builtin function ‘fromJSON’

Fixes #294.