Unary expression `+ <REAL>` panics on `unimplemented!()`

[mhasel]

Describe the bug
Unary expressions with an explicit + operator on REALs will panic in

rusty/src/codegen/generators/expression_generator.rs

Line 400 in b357a05

_ => unimplemented!(),

The same expression with INTs is fine.

To Reproduce
This came up while porting a project and the MRE boils down to this:

FUNCTION main: DINT
VAR
    r: REAL;
    i: INT;
END_VAR
    i := + 6; // this is fine
    r := + r; // hits `unimplemented!()` in `expression_generator.rs:400`
    r := + 6.0; // same as above
END_FUNCTION

Expected behavior
the expression + 6.0 behaving just like 6.0

[volsa]

Related PR #640