This PR
* removes the crate `askama_derive_standalone`,
* makes `askama_derive` a normal library, and
* adds the proc-macro crate `askama_macros`,
Before, it was not possible for another crate to re-export
`askama::Template` in a useful way, because the generated code assumes
that it has access to an `extern crate askama`.
`askama_derive` will export the function `derive_template()` like
`askama_derive_standalone` did, but it has an additional argument to
accept a `TokenStream` that should contain (an) statement(s) to define
the identifier `askama`, e.g. `quote! { extern crate askama; }`.
The new proc-macro crate `askama_macros` now defines the derive-macro
`Template` by calling `askama_derive::derive_template()`.
Prior art: [`encase`] → [`encase_derive`] → [`encase_derive_impl`];
[2298a3e].
[`encase`]: <https://crates.io/crates/encase/0.11.0>
[`encase_derive`]: <https://crates.io/crates/encase_derive/0.11.0>
[`encase_derive_impl`]: <https://crates.io/crates/encase_derive_impl/0.11.0>
[2298a3e]: <2298a3efd5>
This is the same as in e.g. `serde` with the feature of the same name.
If you manually depend on `rinja_derive`, then you can make better use
of your multi-core setup.
