Import and forward-declare classes as needed to compile headers

[leptos-null]

What does this implement/fix? Explain your changes.

Enables more headers to be imported by themselves.

Checklist

• New code follows the code rules
• I've added modulemaps for any new libraries (e.g. see libactivator/module.modulemap): it should be possible to @import MyLibrary; in ObjC, or import MyLibrary in Swift.
• My contribution is code written by myself from reverse-engineered headers, licensed into the Public Domain as per LICENSE.md; or, code written by myself / taken from an existing project, released under an OSI-approved license, and I've added relevant licensing credit to LICENSE.md

Does this close any currently open issues?

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Any relevant logs, error output, etc?

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Any other comments?

This unblocks #72

Where has this been tested?

Operating System: Ubuntu, macOS

Platform: …

Target Platform: iOS

Toolchain Version: …

SDK Version: iOS 14.5