Add field attributes (aliases, ignore, rename) for derive FormatArgs

[smith61]

Implements support for field attributes when using #derive(FormatArgs) on a struct. Adding these attributes allows for hiding of internal fields, or providing backwards compatible names for given fields.

aliases - Comma separated list of alternative names for the provided field.
ignore - If set, field is hidden from format strings.
rename - Overrides the default name for a given field.

This pull request also refactored most of the generation code. While the generated code mostly matches, regular structs now also support indices similar to tuple structs and tuple structs can have named fields using the added attributes. This was done to unify the code and make it agnostic of struct type.

[smith61]

Ideas for possible future attributes:

• Virtual fields of a struct
• Rename all to auto rename all non-ignored fields (Similar to serde's rename_all attribute)
• Override getter for a field
• Override get_child/as_usize for a field

[SpaceManiac]

Thanks for the PR! These features look really useful, and mirror serde's quite nicely. Due to the size of the changes and the complex code, it might take me some time to review.