feat: impl Generator and Controller

[lxl66566]

This PR is a recommitted PR, split from total modification. Actually the generator part had a lot of refactoring since then on, this is the current version, but maybe not a final version.

Depends-On: #8

Each Generator has an id, a ref to global context and a sequence of (Op, DelayStrategy). Each generate will delay a period by its DelayStrategy, and return Op. Generators can filter, map, split-at, take (only support common functional calculator), like a normal iterator.

Each GeneratorGroup has a set of generators, each generator comes with a Counter. Counter indicates the generation ratio between them. Ex. If generator A set Counter to 100 and B set to 1, it means that A will generates 100 Ops, then B generates 1, then loop.
GeneratorGroup also has a GeneratorGroupStrategy to define how to switch the next generator to be used.

Trait DelayAsyncIter provides the ability to get Op (or DelayStrategy) without actually delay. This is useful in testing and chaining generators.