Eliminate global variables associated with builtin IO

This change eliminates global variables like stdout_buffer. Instead we wrap up
the IO information into a new struct io_streams_t, and thread that through
every builtin. This makes the intent clearer, gives us a place to hang new IO
data, and eliminates the ugly global state management like builtin_push_io.

src/input.cpp

@@ -339,7 +339,7 @@ static void input_mapping_insert_sorted(const input_mapping_t &new_mapping)
 }
 
 /* Adds an input mapping */
-void input_mapping_add(const wchar_t *sequence, const wchar_t **commands, size_t commands_len,
+void input_mapping_add(const wchar_t *sequence, const wchar_t * const *commands, size_t commands_len,
                        const wchar_t *mode, const wchar_t *sets_mode)
 {
     CHECK(sequence,);