Locally disable the preparser with the suffix r

It's not really disabling the preparser (the input is still preparsed), but telling the preparser not to process some of the (numerical) input by marking this input as raw (by appending the letter r).

```
sage: type(12)
<type 'sage.rings.integer.Integer'>
sage: type(12r)
<type 'int'>
sage: type(42.42)
<type 'sage.rings.real_mpfr.RealLiteral'>
sage: type(42.42r)
<type 'float'>
```

Also works for Python complex numbers:

```
sage: type(1j)
<type 'sage.rings.complex_number.ComplexNumber'>
sage: type(1jr)
<type 'complex'>
```

It's a bit similar to specifying some strings as raw by prepending an r to `'...'`

or `"..."`

or `'''...'''`

or `"""..."""`

; for instance in `'\t'`

the backslash-t produces a tab, but in `r'\t'`

it stays backslash-t.