This patch adds two new SAWCore primitives:

* `appendString : String -> String -> String`, which appends the underlying
  `Text` values in a `String`, and
* `bytesToString : (n : Nat) -> Vec n (Vec 8 Bool) -> String`, which converts a
  Cryptol string (a sequence of 8-bit ASCII characters) to a SAWCore `String`.

Moreover, this reimplements `ecError` in terms of
`appendString`/`bytesToString` such that invoking the Cryptol `error` function
from SAW will preserve the string passed to `error`. Previously, if you invoked
the following in SAW:

```
sawscript> prove abc {{ error "Descriptive error message" : Bit }}
```

You would get:

```
Run-time error: encountered call to the Cryptol 'error' function
```

Now, you get:

```
Run-time error: encountered call to the Cryptol 'error' function: Descriptive error message
```

Fixes #1326.