Please, drop nix* prefix from function names

[Anton-Latukha]

For example - a lot of functions inside Nix module are named nix*.

There is nixEval in Nix & eval in Eval. But nixEval really runs withNixContext from Builtins as evaluator. If it is confusing - because it is.

withNixContext is really execInsideNixContext, it actually does just a lot of pushScopes, functions in Nix then are also really exec.. ... actually I think dictionary should be expended there (maybe from interpreter/compiler theory), there are a lot of different eval & exec that do different eval & exec - it should be distinquished.

Nix (C++) instantiation-related function in the project also use nix* prefix, but in this case, it is more understandable.