v4.0.0

• Rewrite units to be exponent-based, making things like km^5 and sqkm^-1 possible
• Add generic unit multiplication and division, such as 1kg/1m and 1m*1kg
• Add generic unit combinations, such as 1kg/m, 1km per year and m+km
• Add generic unit simplification. For example, 1kg*1sqm/1s/1s/1s/1a now becomes 1 volt
• Add ≈ indicator when a result is approximate
• Add case-sensitive Bps units for bytes per second
• Use max precision for inexact unit conversions
• Fix ^ allowing numbers to be raised to the power of units