Function minitt::type_check::update_gamma

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

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

Cow is used to simulate immutability.