Scrypt - let expressions

Scrypt should probably have a statement equivalent to the usual `(let [...] ....)` form in Lisp

Examples:

```
let (x=1) x+2;
let (a=1, b=a+1) {a * b;};
(1 + let (b=10) b*b)
```

Might need to think a bit about whether semicolons are mandatory. Probably not? We should perhaps think of this as a let *expression* rather than a *statement*?