Renamed "source" to "origin", and "size" to "length".
Renamed "source" to "origin" in vstack_t struct.
Made vstack functions better indicate malloc failures.
Added vstack_reset() function.
Started renaming of vsbuffer to vsbuilder.

Changed file names only.
Finished name change from vsbuffer to vsbuilder.

Also changed names of types in files.

Since vsbuilder is to be used for building strings, the new name seemed more suitable.
Changed name of vsbuilder_t member "string" to "offset".

The new name better reflects the function of the member.
Changed name of vsbuilder_t member "source" to "origin".
Added vsbuilder_discards() function.
Made vsbuilder functions better indicate malloc failures.