libexpr: allocate ExprPath strings in the allocator

[Radvendii]

Motivation

See this tracking issue for big-picture plan and motivation

Where does this fall in that picture?

• Removes 24-32 bytes from each ExprPath (size of std::string)
  • This is not very much. ~500KB
• Cut excess allocations off the ends of strings
  • This is more than I would have expected. ~5MB (unless there's some other memory reduction I'm not accounting for)
• Once Exprs live in a std::vector, they will need to be trivially moveable. Because of the small string optimization of std::string, moving an ExprPath invalidates its Value's pointer.

Note that we've added a bunch of copies that weren't necessary before. This doesn't seem to impact performance. In the future, we can address this by passing the allocator down into functions like getHome() and having them allocate directly there instead.

Rant about plans for removing the `accessor` field as well

I would like to also remove `ref accessor`. It does take up 16 needless bytes, but the bigger reason is that eventually I want `ExprInt`, `ExprFloat`, `ExprString`, and `ExprPath`, all the "literals" to be referred to **as** Values directly (See tracking issue's todo item "Combine `Expr`s and `Value`s into one address space...") This would remove a layer of indirection and the need to store the Expr at all, but it requires that they not have any other fields (i.e. `accessor`).

The only thing accessor is doing is incrementing/decrementing a refcount for the SourceAccessor. I scoured the internet and could not find a reasonable way to manually increment / decrement the refcount of an std::shared_ptr. I did come up with a janky way that involves using memset and memcpy to skip constructors/destructors, but that seemed like a bad path to go down.

I'm now thinking boost::intrusive_ptr is the way to go here, but I think we should tackle that down the road when it actually becomes relevant.

Context

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[Mic92]

cc @NaN-git @xokdvium

[edolstra]

We can remove the accessor field entirely. It was introduced to support lazy trees, but it's no longer needed there since we mount every input on top of /nix/store using MountedSourceAccessor. So almost every path is using rootFS anyway.

I have an unfinished PR stashed away somewhere that does this. I'll have a look at reviving it.

[Radvendii]

I'm working on reviving Eelco's changes (thanks!) In the meantime that shouldn't impact this PR.