Fixed subtraction and added GCD for Linked Bytes

• Added a trait + convenience function for GCD (greatest common divisor) and implemented the Euclidean algorithm for Linked Bytes.
• Fixed Linked Bytes subtraction. In prior versions, possibly since 0.0.3, subtracting any kind of number from Linked Bytes (which includes both Linked Bytes-encoded numbers themselves as well as built-in integer types) resulted in the last byte being a link byte rather than an end byte if any bytes had to be removed (for example, subtracting 2 from 129).

LBString conversion

• Implemented some traits for LBStrings, allowing for quick conversion to and from Strings.

Linked Bytes string storage

• Added LBString, which is an owned string type implementing the Linked Bytes Unicode encoding, allowing for efficient storage of Unicode strings which is under all circumstances more compact than all current UTF encodings
• Added LBNumRef addition, which was missing due to a copypasting mistake

Minor arithmetic improvements

• Implemented arithmetic with self as the right operand for Linked Bytes, removing the need to always explicitly write op1 + &op2 (same for other operators)
• Fixed the Crates.io badge to link to bigbit instead of bitbit
• Added usize to the list of primitive integer types for which Linked Bytes arithmetic is implemented
• Added the DivRem and DivRemAssign traits for combined division (one operation to get the quotient and remainder)
• Renamed HeadByte::from_headbyte_and_followup into from_raw_parts (breaking)
• Implemented logarithm with arbitrary base for Linked Bytes
• Implemented string formatting for Linked Bytes

Refactor, Linked Bytes multiplication and division, LBNumRef

• Implemented Linked Bytes multiplication and division
• Implemented Linked Bytes arithmetic with primitive integers
• Reorganized the internal module structure
• Added LBNumRef
• Implemented some more traits on LBNum

Linked Bytes subtraction and functional addition

• Implemented Linked Bytes subtraction
• Started the reorganization
• Fixed addition bugs