Adopt more Rust idioms in the library

[sappenin]

Some places in the library don't conform to typical Rust idioms. For example, see Request...

pub trait Request<'a> {
    fn get_common_fields(&self) -> &CommonFields<'a>;
    fn get_common_fields_mut(&mut self) -> &mut CommonFields<'a>;
}

...which uses get_ prefix in non-idiomatic ways.

Standard Rust Accessor Naming Conventions

• field(&self) -> &T: Immutable: reference accessor (no get_ prefix)
• field_mut(&mut self) -> &mut T: Mutable reference accessor.
• into_field(self) -> T: Consuming getter that takes ownership.
• as_field(&self) -> &T or to_field(&self) -> T: Standard type-conversion accessors.

When get IS Idiomatic

The get prefix or method name is used in Rust under limited, specific circumstances:

• Lookups: Methods that require arguments to search or retrieve an item, often returning an Option or Result (e.g., HashMap::get(&key) or Slice::get(index)).
• Single-value wrappers: Smart pointers or concurrency primitives where obtaining the underlying value is the container's core operation (e.g., Cell::get() or RefCell::get_mut()).