Rename as.*() → as_*()

[GegznaV]

I suggest renaming hyperSpec specific as.*() functions by replacing . with _

Current name		Rename to	Alternative	Note
as.wide.df	→	as_wide_df	as_df_wide()	as_df_*() makes the 3 functions to have the same prefixed pattern
as.long.df	→	as_long_df	as_df_long()
as.t.df	→	as_t_df	as_df_t()	what does t mean? Tall? Transposed? @cbeleites
as.hyperSpec	→	as_hyperSpec