Allow customizing the name of the generated WIT type

[jvff]

Motivation

Currently Witty generates types with names that are kebab-case conversions of the Rust names. It would be useful to allow overriding this behavior when desired, similarly to what serde provides when renaming fields.

Proposal

Update the WitType, WitLoad and WitStore derive macros to obtain the WIT name by trying to read it from a name attribute, and falling back to generating the kebab-case name of the Rust type name if the attribute isn't found.