Fix #1069. Non-callable literals shouldn't compile

This is the most aggressive of the proposals, but also most consistent - no apparently non-callable literals are allowed to be used as functions.

Good news:
This also fixes bugs such as:
```
do "hello"
```

Bad news:
I couldn't get interpolated strings to not be callable non-implicitly. This is because the information about them being a string literal is lost during rewrite.

I suppose the test could be moved to test/compilation.