rename "callback" variables/parameters

[milyin]

Describe the release item

The structures are named "closure" and the closure have fields "context", "callback" and "drop"
@YuanYuYuan noticed that as the function's parameters and variable names are named "callback" too, this confuses user.

Maybe we should consider some renaming -e.g. name this parameter "closure" ?

[milyin]

After some discussion month ago the good soultion was not found. Since the word "callback" is used in Rust API, better to not to do renaming of the parameter