Add nix repl :probe

[roberth]

Motivation

For checking laziness (to a degree)

$ src/nix/nix repl -f ~/nixpkgs
Added 23851 variables.

nix-repl> :probe hello
a thunk

nix-repl> :probe stdenv
a set

nix-repl> :probe pythonPackages
a thunk

nix-repl> :probe python39Packages
a thunk

A thunk can be very superficial, but it's at least something

nix-repl> x = true

nix-repl> y = const true null

nix-repl> x
true

nix-repl> :probe x 
a Boolean

nix-repl> :probe y 
a thunk

TODO

• tests

Ideas

Could be extended to provide information about the thunk, such as the kind of thunk (expression, delayed application, blackhole), and/or the relevant source locations for each.

Context

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