chore(library/coercion): cleanup
feat(frontends/lean): allow multiple coercions from class A to B, closes #187

See new tests (for examples)
tests/lean/run/coe10.lean
tests/lean/run/coe11.lean
tests/lean/run/coe9.lean

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): allow multiple coercions from class A to B, closes #187