This makes the removal vector a vector of Symbols instead of Attrs, and
uses a custom Compare to still be able to std::set_difference them.
std::ranges::set_difference **should** be the perfect function for this,
but because for some reason it spuriously requires
`std::indirectly_copyable<I2, Out>`, I can't use it here. This
defficiency has bee recognized before [here](https://github.com/cplusplus/papers/issues/1021),
but no one has driven the fix forward.
Part of #744
Change-Id: I6d2c016ea41e033bf38836f859541b506a6a6964
skye
83bca23d4a