Improve conversions of types in the std lib

[SwayStar123]

There are many missing pairs of conversions that are possible but not implemented in the standard library, aswell as some non trait implementations which are duplicates of their trait implementation counterparts.

• Resolve duplicate functions #6712
• From< u8 > for u16
• From<u8, u16> for u32
• From<u8, u16, u32> for u64
• From<u8, u16, u32, u64> for U128
• From< U128 > for u256, b256
• TryFrom < U128> for u64, u32, u16, u8
• TryFrom< B512 > for b256, u256

Remove duplicate functions

There are implementations such as

impl u16 {
    pub fn try_as_u8(self) -> Option<u8> {
        if self <= u8::max().as_u16() {
            Some(asm(input: self) {
                input: u8
            })
        } else {
            None
        }
    }
}

which serve to attempt conversion of a u16 to a u8, however, there is already an implementation of

impl TryFrom<u16> for u8 {
    fn try_from(u: u16) -> Option<Self> {
        if u > u8::max().as_u16() {
            None
        } else {
            Some(asm(r1: u) {
                r1: u8
            })
        }
    }
}

which serves the same purpose, and with the generic auto implementation of TryInto via

impl<T, U> TryInto<U> for T
where
    U: TryFrom<T>,
{
    fn try_into(self) -> Option<U> {
        U::try_from(self)
    }
}

renders the try_as_* functions redundant.