Upgrade hoedown to 3.0.4

[Eljay]

Some hoedown FFI changes:

• HOEDOWN_EXT_NO_INTRA_EMPHASIS constant changed.
• Updated/tidied up all callback function signatures.
• All opaque data access has an additional layer of indirection for some reason (hoedown_renderer_data).

This also fixes #27862.

[rust-highfive]

r? @brson

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[tamird]

while you're here, this should probably be an empty enum

[daboross]

Isn't c_void basically an empty enum? I mean I guess it's represented in hardware by a u8, but what does the difference make?

[tamird]

i believe that c_void is inhabitable; that is, you can create one

[alexcrichton]

The actual representation or whether this can be constructed isn't so much the problem here, but rather *mut hoedown_document and *mut c_void are the same types. In Rust it's more idiomatic to have those be separate types (to get at least some level of type safety). Creating an empty enum (enum hoedown_document {}) should serve this purpose

[tamird]

opaque should probably also be an empty enum, but that might be more work

[rkruppe]

It's a void * in the C code as well, so I think this is fine (or even more appropriate, since pointers to empty enums aren't represented as *i8 in LLVM, while *mut c_void is).

[tamird]

ditto

[Eljay]

Changed hoedown_document to be an empty enum and squashed commits.

Adding any kind of type safety to the other void pointers is non-trivial because they point to different types in different passes. A lot of this module is very unsafe/hacky but hopefully we can replace it all with a pure rust Markdown parser at some point anyway!

[alexcrichton]

@bors: r+ efc98fa

Thanks!

[bors]

⌛️ Testing commit efc98fa with merge fd7344c...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt