Cache parameter values

[kalekundert]

Currently, parameters are recalculated from scratch on each access. This is pretty wasteful, especially if a lot of calculation goes into the parameter (e.g. expensive cast function). Since parameters aren't really meant to be dynamic, I think it might make sense to always cache values after load(). I would want to have the option to disable the cache on a per-param basis.

Some pseudo-code:

def load(obj):
    ...
    get_meta(obj).cache_version += 1
    ...

class param:
    def __init__(self, ..., dynamic=False, ...):
        self.dynamic = False
        self.cache_version = -1

    def __get__(self, obj):
        meta = get_meta(obj)
        if self.cache_version == meta.cache_version and not self.dynamic:
            return self.cache_value
        else:
            ...
            self.cache_version = meta.cache_version