Deci field context

# A literal takes its type from where it lands (types.md): a struct field's type and an enum case's data
# type are its context, as a declared variable's is, so `0.45` into a `deci64` field is a deci.
struct Price { amount: deci64 }
enum Pay { cash, voucher: deci64 }
MAIN {
    let 'p' := Price[amount = 0.45];
    let 'v' := Pay.voucher[5.00];
    std::print.useful['p'.amount, " ", 'v', newln];
}