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 - needed for ErrorPage
• cot_core::error::error_impl::* - to allow implementing of errors in cot
• Error.backtrace() - needed for supplying error backtrace in ErrorPageTemplateBuilder
• Backtrace.frames() - needed for the Template derive for ErrorPageTemplate (used in error.html template)
• StackFrame.symbol_name() - as above
• StackFrame.location() - as above
• StackFrame - as above
• Body.axum() - needed for request_axum_to_cot in project.rs
• InvalidContentType
• AppName
• RouteName
• BoxRequestHandler - used in openapi.rs and router.rs
• into_box_request_handler - used in method.rs and router.rs

Stayed private with following gotchas:

• NotFound.router() - removed deprecated NotFound constructor methods from Error