The symbol separator cannot be escaped when parsing symbols

Trying to read a symbol containing a dot fails.
```
lurk-user> 'a.b
```

This should yield the symbol `.lurk-user.a.b` — which should print as `a.b` from the `lurk-user` package.

Some more test cases:
```
lurk-user> '.a.b
```
should read as `.a.b` (absolute rather than relative).

And the dot should be escapable, so
```
lurk-user> 'a\.b
```
reads as `|a.b|`.