Update syn: v2.0

[ranfdev]

closes #1061

Problem
syn 2.0 removed the type NestedMeta, that was extensively used by gtk-rs-core to parse macro attributes, without providing a direct replacement type.
Instead, the suggested way to parse an attribute is now to use Attribute::parse_nested_meta, which iterates over the arguments of an attribute and forces you to parse them. parse_nested_meta tracks the spans of the arguments being parsed, so in the case an error is returned, it provides good error messages.

Most of the glib-macros depended on utility functions defined in glib-macros::utils. Those functions often tried to retrieve a single argument inside an attribute, without caring about the other arguments. This is not possible when using Attribute::parse_nested_meta, because it requires to handle every possible argument in a single place. We can't just parse the argument name = "value" and skip nick = "nick", because parse_nested_meta will consider nick as an unexpected value and will throw an error.

Proposed solution
Rewriting the parsing of attributes taking advantage of parse_nested_meta, which is now the suggested way to parse attribute arguments (also see https://docs.rs/syn/latest/syn/meta/struct.ParseNestedMeta.html). A new set of utilities have been implemented in glib-macro::utils to ease the parsing process.

To do
implement some tests for the new parsing utility functions, to polish their interface and ensure correctness.

Observations
I would consider using the crate https://github.com/jf2048/deluxe (by @jf2048, #970) to handle attribute parsing.

Conclusions

• Improvements from syn 1.0:
  • We are up to date!
  • the errors from the macro should now be improved, because parse_nested_meta keeps track of argument spans and ensures every argument is parsed correctly.
• Drawbacks: even though the tests are passing, rewriting the parsing system can easily introduce some bugs. I fear I've missed some corner cases.

[ranfdev]

Ah, I've noticed I still need to change some little things, please don't merge yet.