Move read/write methods from `Function` to `Symbol`

**Is your feature request related to a problem? Please describe.**
Symbols can contain data even if they can't be represented as a function, but the read/write methods for the data of the symbol are currently on the `Function` struct.

**Describe the solution you'd like**
We should move or duplicate them on `Symbol`.

**Describe alternatives you've considered**
N/A
