build: Fix error in dispatch generator for functions with channel_id
api: Rename find_{buffer,window,tabpage}

They were renamed to find_{buffer,window,tabpage}_by_handle to avoid conflicts
with existing functions of the same name.
api: Change type of event data to `Object` from `typval_T`
api: Enable usage of Arrays and Dictionaries as [k]vectors

- Rename a/n/m to items/size/capactity in kvec.h
- Add capactity field to Arrays/Dictionaries
api: Add helper macros for dealing with API type casts