Support for arithmetic shift expressions

When masking specific bits, it's handy to be able to write things in terms of shift expressions:

```s
ld var1
and #(1 << 3) ; mask out bit 3 of 'var1'
st var2
```

But waterbear reports an error like the following for such programs:

```
[ERROR] Failed to assemble shift.s

✘ Expected to find ')'

shift.s:1:8-9
  and #(1 << 3)
          ^
```