better-as 0.1.0

Explicit type casting
Documentation 
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pub trait ExtendingCast<U> {
    fn truncating_cast(self) -> U;
}

macro_rules! impl_cast {
    ($($lhs:ty => $rhs:ty, $f:ident;)+) => {
        $(
            impl ExtendingCast<$rhs> for $lhs {
                #[inline]
                fn truncating_cast(self) -> $rhs {
                    self as $rhs
                }
            }
        )+

        #[test]
        fn size_lt(){
            use core::mem;
            $(
                assert!(mem::size_of::<$lhs>() < mem::size_of::<$rhs>());
            )+
        }

        $(
            #[inline]
            #[must_use]
            pub const fn $f(x: $lhs) -> $rhs{
                x as $rhs
            }
        )+
    };
}

impl_cast!(
    u8  => u16  , u8_to_u16     ;
    u8  => u32  , u8_to_u32     ;
    u8  => u64  , u8_to_u64     ;
    u8  => u128 , u8_to_u128    ;
    u8  => usize, u8_to_usize   ;
    u16 => u32  , u16_to_u32    ;
    u16 => u64  , u16_to_u64    ;
    u16 => u128 , u16_to_u128   ;
    u32 => u64  , u32_to_u64    ;
    u32 => u128 , u32_to_u128   ;
    u64 => u128 , u64_to_u128   ;

    i8  => i16  , i8_to_i16     ;
    i8  => i32  , i8_to_i32     ;
    i8  => i64  , i8_to_i64     ;
    i8  => i128 , i8_to_i128    ;
    i8  => isize, i8_to_isize   ;
    i16 => i32  , i16_to_i32    ;
    i16 => i64  , i16_to_i64    ;
    i16 => i128 , i16_to_i128   ;
    i32 => i64  , i32_to_i64    ;
    i32 => i128 , i32_to_i128   ;
    i64 => i128 , i64_to_i128   ;
);