Persist-to-disk

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1. Added the json serialization mode. This could be specified by `hash_method` when calling `persistf`.
2. If a function is specified to be `cache=ptd.READONLY`, no file lock will be used (to avoid unncessary conflict).

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1. `lock_granularity` can be set differently for each function.
2. Changed the default cache folder to `.cache/persist_to_disk`.

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1. Changed the behavior of `switch_kwarg`. Now, this is not considered an input to the wrapped function. For example, the correct usage is

ptd.persistf(switch_kwarg='switch')
def func1(a=1):
print(1)
func1(a=1, switch=ptd.NOCACHE)

Note how `switch` is not an argument of `func1`.
2. Fix the path inference step, which now finds the absolute paths for `project_path` or `file_path` (the path to the file contaning the function) before inferencing the structure.

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1. Added `set_project_path` to config.