Add BitMap trait for primitive types

[winstonallo]

This trait defines the following API:

pub trait BitMap<T> {
    /// Gets the bit at position `index` from `&self`.
    fn get_bit(&self, index: u8) -> T;
    /// Sets the bit at position `index` in `&self`.
    fn set_bit(&mut self, index: u8, value: T);
    /// Gets the bits at positions `indices.start..indices.end` from `&self`.
    fn get_bits(&self, indices: Range<u8>) -> T;
    /// Sets the bits at positions `indices.start..indices.end` in `&self`.
    fn set_bits(&mut self, indices: Range<u8>, value: T);
}

Also adds a runtime assertions in bitmap structs that panics when the value passed to a setter overflows the field's bit width.