Before we start writing a lot of Expr code.
- `Expr.pi` does not make sense anymore.
- `Expr.elet` is weird. `«let»` is too inconvenient to write. So, I
used `letE` short for `letExpr`. GHC avoids this issue because
keywords are lowercase and constructors are capitalized.
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
```
