fix: improve LessThan and adds assert

[kobigurk]

LessThan at the moment uses Num2Bits(2*n+1), while it practically needs just an extra bit, so it could use n+1 (h/t @jbaylina for noticing that). This is true because the inputs are assumed to be less than or equal to n bits, and so the difference of a and b + 2^n, if a < b, will have the n+1 bit off, which is what we check.

Additionally, n must be <= 252, since Num2Bits can have multiple correct witnesses for num bits >= 254.

[jbaylina]

Please add the assert.

[kobigurk]

It's there!