The code for serialization Expr nodes back into (pseudo-)Nix has been
removed for being subtly error-prone and tedious to maintain. Instead,
`nix-instantiate --parse` now prints a JSON representation of the AST.
Usage patterns of the --parse flag I've found in the wild:
1. Check if a file is well-formed, i.e. discard output and test exit code
2. Get parser errors from a file, i.e. discard stdout and use stderr
3. Nixfmt uses --parse to test equivalence pre/post format, and that property is (should be?) preserved
None of these should break with the current change
Closes#487
Change-Id: Icdbaad17790f2ad8765fa08e02e6597ee4c7a909
piegames
cf57b5c14c