Shell: Cache PATH for faster tab completion

This patch reduces the O(n) tab completion to something like O(log(n)).
The cache is just a sorted vector of strings and we binary search it to
get a string matching our input, and then check the surrounding strings
to see if we need to remove any characters. Also we no longer stat each
file every time.

Also added an #include in BinarySearch since it was using size_t. Oops.

If `export` is called, we recache. Need to implement the `hash` builtin
for when an executable has been added to a directory in PATH.

Shell/main.cpp

@@ -64,7 +64,12 @@ static int sh_export(int argc, char** argv)
         return 1;
     }
 
-    return setenv(parts[0].characters(), parts[1].characters(), 1);
+    int setenv_return = setenv(parts[0].characters(), parts[1].characters(), 1);
+
+    if (setenv_return == 0 && parts[0] == "PATH")
+        editor.cache_path();
+
+    return setenv_return;
 }
 
 static int sh_unset(int argc, char** argv)
@@ -910,6 +915,8 @@ int main(int argc, char** argv)
     load_history();
     atexit(save_history);
 
+    editor.cache_path();
+
     for (;;) {
         auto line = editor.get_line(prompt());
         if (line.is_empty())