Theorem list.length_erase_of_not_mem

feat(*): working on list/basic, robusting simp proofs

fix(data/finset): use the type class projections for insert; hide most constants using protected; add image of finset

Merge branch 'master' of https://github.com/leanprover/mathlib

fix(algebra/group): add every transport theorem from main Lean repository

refactor(data/finset): fix formatting issues

feat(topology): add topological structures for groups, ring, and linear orders; add instances for rat and real

feat(data/finset): add basics for finsets