remove dlopen hack (#1468)

Resolves #1369

Motivation of this PR:
* no need to compile `cargo-pgrx` after upgrading compiler
* make it possible to support `binstall`

This PR:
* recompilation occurs even if we use a nightly toolchain
* schema is generated by `cargo run --bin pgrx_embed_${ext_name}` ,
instead of using `libloading` crate (unused symbols are removed by
linker, so it compiles)
* removes compiler version check
* removes codes about stub
* removes unnecessary metadata-related traits

This is a breaking change: users need to write `pgrx::pgrx_embed!();` to
`src/pgrx_embed.rs` and add that `[[bin]]` to the `Cargo.toml`,
otherwise schema generation will report an error.

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Signed-off-by: usamoi <[email protected]>