feat: add fixed-size unsigned integer for ECLAIR

[tsunrise]

Add AssertWithinRange trait and U128 type for ECLAIR.

Closes: #141

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[bhgomes]

@BoyuanFeng and I are going to rewrite some of this (and add some bit-decomposition optimizations).

[tsunrise]

@BoyuanFeng and I are going to rewrite some of this (and add some bit-decomposition optimizations).

Sounds good, thanks!

[bhgomes]

Some comments on the tests. Also, @BoyuanFeng don't forget the following:

• Add generic implementation for UnsignedInteger from yesterday's discussion
• Lets also add an unwrapping method for UnsignedInteger:

      pub fn into_inner(self) -> T {
          self.0
      }