Function eval_let 

pub fn eval_let(
    cx: &mut Cx,
    outer: &LexicalEnv,
    bindings: Vec<(Symbol, Value)>,
    body: impl FnOnce(&mut Cx, &LexicalEnv) -> Result<Value>,
) -> Result<Value>

Evaluates a let form: parallel bindings in a fresh child scope.

All values are supplied up front, so no binding can observe another; body then runs in the child scope.