This is a version of Expr::eval that doesn't necessarily have to
evaluate the value into a non-thunk, while returning a specific
attribute

The evalAttr of expressions that evaluate to a subexpression should call
evalAttr on that subexpression, therefore bubbling up any potential
thunks

This introduces an expression base class that can be used for lazy
binary operations, along with a value type for storing partial results
of such expressions

This makes the // operator lazy in attribute names, only evaluates what
is necessary to get a specific attribute.

So as to not increase the sizeof(Value) from 24 bytes to 40 bytes

I think this is needed so that any variables on the sides get updated
properly

Without this,

  bin/nix-instantiate ~/src/nixpkgs/nixos --arg configuration ~/src/nixpkgs/nixos/modules/profiles/demo.nix -A vm

fails with a segfault

Not sure why this doesn't happen with the commit that makes // lazy
though

The previous implementation relied on uninitialized memory not being a
certain value for it to work

Since it throws inf rec when it doesn't have to sometimes

This time it also works with lazy binops

The previous optimizations for prevention of allocation had to be undone

Code still needs cleanup, but it should be sound

With the previous commit, passing left/right is now unnecessary

Every evaluation can now pass a handler, which is called once the
resulting value is either a tAttrs or a tLazyAttrs

There are two handlers:
- One for weak head normal form (changes tLazyAttrs into tAttrs)
- One for getting an attribute (lazily gets attributes from tLazyAttrs,
  strictly from tAttrs)

By using bitfields to encode the left/right blackhole for infinite
recursion detection

…me allocs"

This reverts commit 4a2c47a.