Add callback-variants to C library.

[BrianPugh]

For the one-shot functions, it would be useful for variants of the compress/decompress functions to exist that periodically invoke a user-provided callback. This could be used for kicking a watchdog, updating a progressbar, etc.

typedef int (*tamp_callback_t)(void *user_data, size_t bytes_processed, size_t total_bytes);
tamp_res tamp_compressor_compress_cb(..., tamp_callback_t cb, void *user_data);
tamp_res tamp_compressor_compress_flush_cb(..., tamp_callback_t cb, void *user_data);
tamp_res tamp_decompressor_decompress_cb(..., tamp_callback_t cb, void *user_data);