[Feature Request] More operators and basic functions about typeql

[shreyas1599]

@lveillard has already written a pretty exhaustive list! For my use case, I'd like the arithmetic operations enabled for long and double done on this #260 to work with datetime. Specifically,

• adding days to a given date
• function/arithmetic operation that can subtract two dates and output the number of days/weeks/months/years between the two given dates.
• Or a function that converts the datetime to unix time so that arithmetic operations on long and double can be used