Fix and unify boolean RHS parsing.	

Problem

Scrooge is inconsistent in RHS handling for bool values, e.g.
```
const bool x = NotABool        # Throws an exception
const bool y = 1               # no exception, generates invalid scala like "val y: Boolean = 1"
struct S { bool x = NotABool } # no exception. Translated to true, valid scala generated.
struct S { bool x = false }    # Translated to true (!)
struct S { bool x = 1 }        # Translated to true (current workaround that doesn't work for consts)
```
Solution

Unify the handling of bool RHS values.

Result

Booleans get parsed as expected. Valid values are {true, True, false, False, 0, 1}; other values are not allowed.