chore(emacs/README.md): add 'f' as a required package for lean-mode
fix(bin/linja): show ninja error/warning
feat(emacs/lean-company): auto-completion for import

Close #126
feat(src/cmake/modules/CleanOlean.cmake): add CleanOlean.cmake

Instead of using 'find -delete', use CMAKE's FILE GLOB_RECURSE and FILE
REMOVE to implement clean-olean.

Related issue: #166
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>