Extend performance group with list-length anti-pattern hints

[mgajda]

Three new opt-in warnings in the performance group targeting lazy-list
O(n) spine-forcing anti-patterns that are not covered by the existing
default-group length/null hints.

Summary

• map f (reverse x) -> reverse (map f x) -- extends the existing
  "Move reverse out" family (filter, mapMaybe, catMaybes, lefts,
  rights) to cover map, avoiding one intermediate cons spine.
• head (map f x) -> f (head x) -- drops the fully-allocated mapped
  cons spine when only the first element is consumed.
• last (map f x) -> f (last x) -- same idea for the last element.

The default group already covers length x {==,>,>=,<,<=,/=} {0,1} and
reverse (reverse x); these three hints address the remaining common
spine-forcing patterns.

Independence

This PR is intentionally independent of other in-flight performance
group proposals (STM-avoidance, strictness/roundtrip, IORef/State/
container folds). If one of those merges first, this branch simply
extends the existing performance group rather than introducing it; if
it merges first, those branches extend the group introduced here. The
three rules added here do not overlap in name, LHS, or RHS with any
rule in those other branches.

Activation

hlint --with-group=performance FILE.hs

Test plan

• cabal build succeeds
• hlint --test passes (965 hints, up from 964)
• New test file tests/flag-with-group-performance-length.test
  covers all three rules; exact column ranges verified
• Rules do not fire without --with-group=performance (opt-in)
• No conflict with existing default-group rules on length/null/reverse