Function minitt::check::expr::check_fallback

pub fn check_fallback(
    index: u32, 
    tcs: TCS, 
    body: Expression, 
    signature: Value
) -> TCM<TCS>

$$ \frac{\rho,\Gamma\vdash_l M \Rightarrow t' \quad \textsf{R}_l\ t = \textsf{R}_l\ t'} {\rho,\Gamma\vdash_l M\Leftarrow t} $$ Fallback rule of instance check.
First infer the expression type, then do subtyping comparison.