Expr: remove redundant fields, add nrExprs

[roberth]

Motivation

Minor things I've found while helping out a bit with #8778

• Slim int and float exprs
• Count how many AST nodes we load

Context

Priorities

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[Ericson2314]

Is there a reason why we don't do the same thing for the other exprs, like ExprString?

[roberth]

ExprString

That one seemed to be in an optimum, although possibly a local one of course. It uses a bunch of std::move and it appears that the bytes are already shared.
Removing the std::string from the Expr would remove the thing that owns the bytes. Another solution is perhaps to make it leak those bytes and than refer to it only in the Value. This does not seem like an obvious win, and I'm only doing low hanging fruit in this PR.

The bools are ExprVar, so not much that can be done there.

ExprPath is like string but is subject to change anyway with SourcePath I think.

Didn't spot any other instances of such duplication.

[Ericson2314]

Ah! I didn't realize the std::string owns what the Value points too. OK that makes perfect sense and makes clear rules for when the other field is needed.