Expand `else` to `True` instead of `otherwise`.

Makes `if` syntax independent of the value of `otherwise`.
Includes test case.
Optimize True branch in multi-way if code generation.
Merge pull request #412 from maxsnew/otherwise

Expand `else` to `True` instead of `otherwise`.
Cosmetic: give function a more descriptive name
Change semantics of keepWhen/dropWhen.

They no longer propagate the right Signal when the left Signal
changes.
Uncomplicate keepWhen implementation
Remove old JS dropWhen/keepWhen