Support sign_ext in std.x

Sign extending a value from one width to another is more annoying than it should be. Cases which are particularly annoying are if the to/from type are not signed and when the to/from types are typedefs. For example:

```
type x_t = u8
type y_t = u16

let x = x_t:0xf;
let signext_x = (x as s8) as s16;
```

It'd be nice to be able to simply do:

```
std::sign_ext(x, y_t:0)
```

Or if we supported types as parametric parameters:

```
std::sign_ext(x)
```