added more functionalities to kvec

[Carlos-Mero]

The original implementation of kvec.h is quite old (the last modification is even 12 years before) and lacks a bunch of functionalities. So I made some improvements to the source code.

• I added a statement to kv_resize so that it can correctly handle the changes of (v).n while resizing.
• I added two macros kv_insert and kv_remove as dynamic operations on the vector. They are some basic operations adopted by STL in C++ and other implementations of vector.
• The original kv_pop method didn't decrease the capability of the vector, which could cause memory waste in some cases. So I defined another macro called kv_p to address this problem. The macro kv_pop is kept unchanged for capability, and it can be used for better performance.