libexpr: allocate ExprOpHasAttr's AttrPath in Exprs::alloc

[Radvendii]

Motivation

For big picture motivation, see the tracking issue

This PR moves the data from the vector ExprOpHasAttr::attrPath into our Exprs arena, and then refers to it by a std::span<AttrName>. Unlike for ExprSelect, here we do not benefit from separating out the length and start pointer of the std::span, since alignment would bring us back up to 16 bytes anyways..

• Removes 8 bytes per ExprOpHasAttr by going from std::vector (24 bytes) to std::span<AttrName> (16 bytes)

Context

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