refactor!: extract framework core to cot-core

[seqre]

This PR attempts to extract the core functionality of the framework to separate crate to speed up compilation among other reasons.

The modules fully/partially moved:

• basic Cot types
• body
• error
• headers
• html
• json
• response
• request
• middleware
• handler

I think moving router would be valid move as well, but in the current format it's too intertwined with OpenAPI functionality (that we don't want to pull into core) that it can be done at a later time.

With cot-core being a separate crate, the following items had to become public:

• cot_core::error::backtrace
• cot_core::error::error_impl::*
• Error.backtrace()
• NotFound.router()
• Backtrace.frames()
• StackFrame.symbol_name()
• StackFrame.location()
• StackFrame
• Body.axum()
• Body.wrapper()
• Body.inner
• BodyInner
• InvalidContentType
• AppName
• RouteName
• BoxRequestHandler
• into_box_request_handler

[seqre]

The nightly error should be fixed by rust-lang/rust#150939

[m4tx]

Looks nice, and seems like it lays a solid foundation for our eventual extraction of the ORM to a separate crate!

I've tried to generate the docs with just docs and it seems like there's a ton of warnings coming from cot-core (outside of missing examples, which are expected) - please make sure these are fixed before we consider merging this.

[m4tx]

Can you elaborate more about this? What's the exact issue this is trying to solve?

[seqre]

During refactor cargo would complain about double implementations, but I was able to remove it now and it works. I suppose it was because the refactor was not finished. I removed it.

[m4tx]

Perhaps it makes sense to change this enum variant to be a struct variant with named field rather than enum variant? After all, we might (however unlikely) want to add some more fields to it, at which point we'll probably need to add some naming so it's clear what they do. Think "future-proofing the APIs".