Macro bs_collect_hex16

macro_rules! bs_collect_hex16 {
    ($context:expr, $var:expr, $on_byte:expr, $on_overflow:expr, $on_eos:expr, $ty:ty) => { ... };
    ($context:expr, $var:expr, $on_byte:expr, $on_overflow:expr, $on_eos:expr) => { ... };
    ($context:expr, $var:expr, $on_overflow:expr, $on_eos:expr) => { ... };
}

Collect all sequential hex bytes into $var (u16), and convert them into an unsigned integer. If $on_byte is supplied, for each new byte execute $on_byte. Upon locating end-of-stream execute $on_eos. If an overflow would occur, execute $on_overflow.

Exit the collection loop upon locating a non-hex byte.