Create an arbitrary precision complex number ([dashu_cmplx::CBig]) with base 2 rounding towards zero.
Each coefficient is a base-2 `FBig` literal (the same grammar as [fbig!]). The literal uses the
algebraic `a+bi` notation, or a `re, im` pair:
```rust
# use dashu_macros::cbig;
let z = cbig!(11+100i); // 3 + 4i in base 2
let r = cbig!(111); // purely real (7)
let im = cbig!(10i); // purely imaginary (2i)
let p = cbig!(11, -100); // pair form: 3 - 4i
assert_eq!(z, p + cbig!(1000i)); // (3-4i) + 8i = 3+4i
```