feat!: add Definition::Primitive

[dj8yfo]

this pr takes a step back and attempts to add definitions to all types, supported by BorshSchema,
in order to allow remove comments like

    /// Zero-sized types should follow the convention of either providing a [Definition] or
    /// specifying `"nil"` as their [Declaration] for this method to work correctly.

from various methods

[dj8yfo]

current master results in this fail of test assertion:

---- schema::container_ext::max_size::tests::max_serialized_size_built_in_types stdout ----
thread 'schema::container_ext::max_size::tests::max_serialized_size_built_in_types' panicked at 'assertion failed: `(left == right)`
  left: `Ok(9)`,
 right: `Err(MissingDefinition("Vec<u8>"))`'

[dj8yfo]

thread 'validate_for_derived_types' panicked at 'assertion failed: `(left == right)`
  left: `Ok(())`,
 right: `Err(MissingDefinition("string"))`'

[dj8yfo]

where str and String would be

pub enum Definition {
    /// `special_encoding` of `true` indicates that not all 
    /// sequences of `elements` type, allowed by the type, are valid with respect
    /// to `Sequence` type being defined 
    Sequence { elements: Declaration, special_encoding: bool },
    ...
}
impl BorshSchema for String {
    #[inline]
    fn add_definitions_recursively(
        definitions: &mut BTreeMap<Declaration, Definition>,
    ) {
        let definition = Definition::Sequence {
            elements: u8::declaration(),
            special_encoding: true,
        };
        add_definition(Self::declaration(), definition, definitions);
        u8::add_definitions_recursively(definitions);
        
    }
    #[inline]
    fn declaration() -> Declaration {
        "string".into()
    }
}

or just

        let definition = Definition::Sequence {
            elements: u8::declaration(),
        };