Enum expressions without boundary spaces

The following is valid syntax where both `One` and `Two` are valid enum IDs. 

```rust
enum MyEnum { One = 1Two = 2 }
```

# Issue 

Currently, without a space between `1` and `Two`, Soft-parser generates an AST with the assignment `One = 1Two`.

Narrowing expression parsers in #387 resolves this.