lancet Files

Feature:

1. convertor package: add ToStdBase64, ToUrlBase64, ToRawStdBase64, ToRawUrlBase64 functions.
2. datastructure package: add Optional datastructure, add Reset and refact itertor logic. add FilterByValue for HashMap.
3. function package: add predicate function logic (Or, And, Nor...), add AcceptIf function.
4. fileutil package: add CopyDir, ParallelChunkRead functions.
5. mathutil package: add CeilToFloat,CeilToString,FloorToFloat,FloorToString functions.
6. strutil package: add SubInBetween, HammingDistance functions.
7. slice package: add SetToDefaultIf functions.

Fix:

1. maputil package: fix bug of MapTo(issue [#169]).
2. strutil package: fix bug of After and Before function(issue [#168]).
3. random package: fix MAXIMUM_CAPACITY bug (issue [#166])

Break Change:

1. retry package: Add retry backoff policy.
2. datastructure pacakge: rename constructor of Set (NewSet->New, NewSetFromSlice->FromSlice)，add ToSlice function for Set.
3. mathutil package: refact RoundToFloat, TruncRound, TruncRound.

Thanks @donutloop, @cannian1 , @coloraven to contribute high quality code!