Add dynamic linking support

[axelkar]

Steps:

1. Rename xilem to xilem-internal
2. Create xilem with pub use xilem_internal::*;.
3. Create xilem-dylib with [lib] crate-type = ["dylib"] in Cargo.toml and use xilem_internal; in lib.rs.
4. Add a feature to xilem to enable the xilem-dylib dependency and use xilem_dylib;.

This can also be made work without creating a new xilem-internal package by making package consumers add use xilem_dylib; themselves.

This makes the program depend on Xilem via a dynamic library instead of statically linking:

$ ldd target/debug/myapp | rg xilem
        libxilem_dylib.so => not found
$ du -sh target/debug/myapp target/debug/libxilem_dylib.so
41M     target/debug/myapp
338M    target/debug/libxilem_dylib.so

For me, this reduced incremental compilation times (simple app) from ~1.75s to ~0.95s. That's -45%.

Dynamically linking dioxus-devtools and winit over made the incremental compilation time drop further down to 0.87s and produced this:

$ ldd target/debug/myapp | rg xilem
        libxilem_dylib.so => not found
$ du -sh target/debug/myapp target/debug/libxilem_dylib.so
26M     target/debug/myapp
358M    target/debug/libxilem_dylib.so

Without dynamic linking, the linker can remove more unused code and perform other optimizations.

$ ldd target/debug/myapp | rg xilem
        libxilem_dylib.so => not found
$ du -sh target/debug/myapp
317M     target/debug/myapp

References:

• Add bevy_dylib to force dynamic linking of bevy bevyengine/bevy#808
• https://doc.rust-lang.org/reference/linkage.html
• https://docs.rs/crate/bevy_dylib/latest/source/Cargo.toml.orig
• https://docs.rs/crate/bevy_dylib/latest/source/src/lib.rs
• https://docs.rs/crate/bevy/latest/source/src/lib.rs