Make struct vgr_hex_coordinate into a value type.

Removed the reference counted object machinery from vgr_hex_coordinate.
Added vgr_string_alloc_from_hex_coordinate() function.

Added vgr_memory.h/.c with wrappers for common memory allocating
functions.
Turn trade classification into a normal struct.

Changed vgr_trade_classification_t into struct vgr_trade_classification.

Added vgr_strarray.h/.c with functions for managing string arrays.
Addedd vgr_string.h/.c with string utilities.