base: export default ifThenElse implementation for RebindableSyntax
## Motivation
People making use of `RebindableSyntax` not seeking to rebind the `if` statement are forced to either
explicitly define it or avoid using such statements. Considering many (citation needed) uses of
`RebindableSyntax` only use it to rebind a handful of items, and leave the rest to the default
interpretation, this is especially annoying. Besides which, it seems reasonable that
```
{-# LANGUAGE RebindableSyntax #-}
import Prelude
```
should be a nop at the top level (at the local level, bindings might shadow `Prelude` bindings, but that
behaviour again looks reasonable).
As for taking up name space, users of `RebindableSyntax` already need to fiddle with `Prelude`'s import
list, adding one name to hide/expose shouldn't be a problem.
## Proposal
Export
```
ifThenElse :: Bool -> a -> a -> a
ifThenElse True x _ = x
ifThenElse False _ y = y
```
from `Prelude`
issue