ilog2 0.1.0

#Integer Logarithm Library for Rust

##API List

1. roundup_pow2(x) for u8 ... u64, returns same integer type.
2. rounddown_pow2(x) for u8 ... u64, returns same integer type.
3. bit_length<u8 ... u64>(), returns usize
4. lsb(x) for u8 ... u64, returns i8, -1i8 on error.
5. msb(x) for u8 ... u64, returns i8, -1i8 on error.

• msb: Find first 1 bit from msb position. Return its index (index starts from lsb).
• lsb: Find first 1 bit from lsb position. Return its index (index starts from lsb).