Reduce size of complete_entry_opt_t

It's gone from 136 bytes to a 128 bytes by rearranging the items in order of
decreasing alignment requirements. While this reduces the memory consumption
slightly (by around 6%) for each completion we have in-memory, that translates
to only around ~8KiB of savings for a command with 1000 possible completions,
which is nice but ultimately not that big of a deal.

The bigger benefit is that a single `complete_entry_t` might now fit in a cache
line, hopefully making the process of testing completions for matches more
cache friendly (and maybe even faster).

src/complete.cpp

@@ -92,14 +92,14 @@ namespace {
 struct complete_entry_opt_t {
     /// Text of the option (like 'foo').
     wcstring option;
-    /// Type of the option: args_only, short, single_long, or double_long.
-    complete_option_type_t type;
     /// Arguments to the option.
     wcstring comp;
     /// Description of the completion.
     wcstring desc;
     /// Conditions under which to use the option.
     wcstring_list_t conditions;
+    /// Type of the option: args_only, short, single_long, or double_long.
+    complete_option_type_t type;
     /// Determines how completions should be performed on the argument after the switch.
     completion_mode_t result_mode;
     /// Completion flags.

src/complete.h

@@ -198,7 +198,7 @@ class completion_receiver_t {
     const size_t limit_;
 };
 
-enum complete_option_type_t {
+enum complete_option_type_t : uint8_t {
     option_type_args_only,    // no option
     option_type_short,        // -x
     option_type_single_long,  // -foo