Make enums support the swift_name attrib

[chinedufn]

This commit makes enums support the swift_name attribute, which lets
you choose the name that shows up on the Swift side.

For example, the following is now possible:

#[swift_bridge::bridge]
mod ffi {
    #[swift_bridge(swift_name = "EnumRename")]
    enum EnumName {
        Variant1,
        Variant2,
    }
}

[bes]

This should complete everything I asked about in #122 ?

[chinedufn]

This doesn't add support for swift_name on Rust functions, so #122 is still open.

We essentially need to change this

swift-bridge/crates/swift-bridge-ir/src/codegen/generate_swift/generate_function_swift_calls_rust.rs

Line 62 in 54b9c09

format!("public func {}", fn_name.as_str())

To something along the lines of:

if let Some(swift_name) = &function.swift_name_override {
    format!("public func {}", swift_name.value())
} else {
    format!("public func {}", fn_name.as_str())
}