libsframe: avoid unnecessary type casts

libsframe: avoid unnecessary type casts

Change the data type of some of the members of the sframe_decoder_ctx
and sframe_encoder_ctx data structures to use the applicable data types
explicitly. Current implementation in libsframe does type casts, which
seem unnecessary.

libsframe/
	* libsframe/sframe-impl.h (struct sframe_decoder_ctx): Use
	applicable data type explicitly.
	(struct sframe_encoder_ctx): Likewise. Use same style of
	comments consistently.
	* libsframe/sframe.c (struct sf_fde_tbl): Define without
	typedef.
	(struct sf_fre_tbl): Likewise.
	(sframe_decode): Remove unnecessary type casts.
	(sframe_encoder_get_funcdesc_at_index): Likewise.
	(sframe_encoder_add_fre): Likewise.
	(sframe_encoder_add_funcdesc): Likewise.
	(sframe_sort_funcdesc): Likewise.
	(sframe_encoder_write_sframe): Likewise.