`switch` macro evaluates the match value multiple times

The `switch` macro evaluates the match value once for each case. This can cause duplicate side effects and strange errors if the switch value tries to bind variables.

For example:

```
(import std.Prelude)
(switch (let n 0)
  1 2
  3 4
)
```

This results in the error:

```
MutabilityError: Can not use 'let' to redefine variable `n'
```