derive: refactor parser

[Kijewski]

parser.rs was a single file containing almost 2000 lines.

This PR split the file into multiple, smaller files. Expr, Node, and Target each get an own file. Each struct gets a parse() method that return Result<Self>, and every other detail is private to the file.

This PR should make this essential part of Askama more easy to understand, and make future modifications easier.

[djc]

Good stuff! Some thoughts on the new structure:

• I'd like to drop the parse_ prefix on modules within parser, since it seems a bit stuttery to have both.
• I think we stuff in parse_common could just move into parser/mod.rs directly.
• I'd prefer to move the contents of parse_target into parse_node.
• I'd then also like to move the Expr definition into the expr module and the Node and Target definitions into node, so that we no longer need a separate ast module.

[Kijewski]

• I'd like to drop the parse_ prefix on modules within parser, since it seems a bit stuttery to have both.
• I think we stuff in parse_common could just move into parser/mod.rs directly.
• I'd prefer to move the contents of parse_target into parse_node.

Done.

• I'd then also like to move the Expr definition into the expr module and the Node and Target definitions into node, so that we no longer need a separate ast module.

I was thinking that this could be the starting point of a more universal interface if someone wants to implement another syntax, e.g. handlebars (#395). But I guess that's not going to happen anytime soon (not by me, anyway :) ). If someone were to implement another syntax, then they can move the structs into their own file.

[djc]

Nice, thanks!