Remove unnecessary borrow_parts() methods

Remove unnecessary borrow_parts() methods

Accessing a struct through an std::syn::MutexGuard using implicit
dereferencing can confuse the borrow checker. This situation arises when
obtaining mutable references to more than one field of the struct, which
is normally allowed.

https://doc.rust-lang.org/nomicon/borrow-splitting.html

However, when using implicit dereferencing, a mutable reference to the
the entire struct is taken. Thus, attempting to access another field in
this manner will lead to a compilation error.

https://doc.rust-lang.org/error-index.html#E0499

A simple way to avoid this is to first obtain a mutable reference to the
struct using explicit dereferencing.