We now use `mapM`, `foldlM`, ... like Haskell.
Motivation: fixes the inconsistent naming convetion. We are already
using the `M` suffix for functions such as `anyM`. We used `anyM`
because `many` is a valid English word.
cc @kha @dselsam
Preparation for new unifier.
@dselsam: I had to make minor changes at `Synth.Lean`, and add messy
code to `Context.lean`. `Context.lean` will be deleted in the future.
So, it is not a big deal.
This is just a draft.
```
for f in `find . -name '*.lean'`; do echo $f; gsed "/^import/s/\b\(.\)/\u\1/g" $f > tmp; gsed "/^Import/s/Import/import/g" tmp > $f; done
```
