Function minitt::type_check::update_gamma

pub fn update_gamma<'a, Name: DebuggableNameTrait>(
    gamma: Gamma<'a, Name>, 
    pattern: &Pattern<Name>, 
    type_val: Value<Name>, 
    val: Value<Name>
) -> TCM<Gamma<'a, Name>>

upG in Mini-TT.
Gamma |- p : t = u => Gamma’

However, since Rust is an imperative language, we use mutable reference instead of making it monadic.