Extract a parser crate

[djc]

Fixes #760, obviates #782.

[djc]

@Kijewski (when) will you have time to review this? Rebasing this will be painful.

[Kijewski]

The changes look great! That's a big improvement in legibility and maintainability.

I went through the commits in chronological order, and looked at the final code. The individual commits look fine, and the result is great.

[Kijewski]

Why did you replace TryFrom with TryInto?

Library authors should usually not directly implement this trait, but should prefer implementing the TryFrom trait, which offers greater flexibility and provides an equivalent TryInto implementation for free, thanks to a blanket implementation in the standard library.

[djc]

Coherence. At the end of this PR, Syntax no longer lives in askama_derive, so we're not allowed to implement the foreign trait TryFrom for the foreign type Syntax. However, we can implement TryInto for RawSyntax, since that's local. This is explained in the docs for Into, which the TryInto docs link to.

[Kijewski]

Maybe the simplest solution would be to overwrite the current head with your branch, and backport #837, #838 and #839 afterwards?

[djc]

Maybe the simplest solution would be to overwrite the current head with your branch, and backport #837, #838 and #839 afterwards?

I did it the hard way and properly rebased it instead. :)

[GuillaumeGomez]

Nice, thanks!