removed TODO file, as we use the github issue tracker system
added types for metadata
added helper function prototypes
added function prototypes for metadata helpers
added implementations

added implementations for inline function helpers for metadata stuff and
added implementations without bodies for non-inline functions
fixed missing keyword "struct"
added documentation for non-inline helper functions
fixed function prototypes: The functions do not change the key
added implementation of thessc_data_add_metadata()
added implementation of thessc_data_metadata_find_flat()
added metadata_len member for metadata_t
added lost endif macro
added implementation of thessc_data_metadata_find_deep()
added helper for setting int value in metadata_t
added helper for setting str value in metadata_t
added helper for setting metadata value in metadata_t
added helper for allocating new metadata_t obj
metadata_t key should be constant