Function minitt::check::tcm::update_gamma

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

Move version of upG in Mini-TT.
$$ \frac{}{\Gamma \vdash x:t=v\Rightarrow \Gamma,x:t} \quad \frac{}{\Gamma \vdash _:t=v\Rightarrow \Gamma} $$ $$ \frac{\Gamma \vdash p_1:t_1=v.1 \Rightarrow \Gamma_1 \quad \Gamma \vdash p_2:\textsf{inst}\ g(v.1)=v.2 \Rightarrow \Gamma_2} {\Gamma \vdash (p_1,p_2):\Sigma\ t_1\ g=v\Rightarrow \Gamma_2} $$ Cow is used to simulate immutability.