Feature request: Expose Item cost

[tarmo-randma]

This is related to the recent excellent #152.

For cache monitoring purposes it would be useful to have access to the current cache "cost" or memory consumption. The item count is already exposed with the Len() function so this would be somewhat similar.

Since item cost is atm a private field, it is not even possible to calculate the cost myself by ranging over all values. Or is there some less obious way how this info is exposed atm?

[swithek]

I don't think there would be any problems if this field were exposed through a method like Len(). We'll try to include it in the next release.

[swithek]

Related PR: #173