Multiple memory safety issues in actix-web

Affected versions contain multiple memory safety issues, such as:

• Unsoundly coercing immutable references to mutable references
• Unsoundly extending lifetimes of strings
• Adding the Send marker trait to objects that cannot be safely sent between threads

This may result in a variety of memory corruption scenarios, most likely use-after-free.

A signficant refactoring effort has been conducted to resolve these issues.