imap: use ISALNUM() for alphanumeric checks

This commit replaces a self-made character check for alphanumeric
characters within imap_is_bchar() with the ISALNUM() macro, as it is
reduces the size of the code and makes the performance better, due to
ASCII arithmetic.

Closes curl#9289

lib/imap.c

@@ -83,6 +83,7 @@
 #include "bufref.h"
 #include "curl_sasl.h"
 #include "warnless.h"
+#include "curl_ctype.h"
 
 /* The last 3 #include files should be in this order */
 #include "curl_printf.h"
@@ -1885,22 +1886,17 @@ static char *imap_atom(const char *str, bool escape_only)
  */
 static bool imap_is_bchar(char ch)
 {
+  /* Peforming the alnum check with this macro is faster because of ASCII
+     artihmetic */
+  if(ISALNUM(ch))
+    return true;
+
   switch(ch) {
     /* bchar */
     case ':': case '@': case '/':
     /* bchar -> achar */
     case '&': case '=':
-    /* bchar -> achar -> uchar -> unreserved */
-    case '0': case '1': case '2': case '3': case '4': case '5': case '6':
-    case '7': case '8': case '9':
-    case 'A': case 'B': case 'C': case 'D': case 'E': case 'F': case 'G':
-    case 'H': case 'I': case 'J': case 'K': case 'L': case 'M': case 'N':
-    case 'O': case 'P': case 'Q': case 'R': case 'S': case 'T': case 'U':
-    case 'V': case 'W': case 'X': case 'Y': case 'Z':
-    case 'a': case 'b': case 'c': case 'd': case 'e': case 'f': case 'g':
-    case 'h': case 'i': case 'j': case 'k': case 'l': case 'm': case 'n':
-    case 'o': case 'p': case 'q': case 'r': case 's': case 't': case 'u':
-    case 'v': case 'w': case 'x': case 'y': case 'z':
+    /* bchar -> achar -> uchar -> unreserved (without alphanumeric) */
     case '-': case '.': case '_': case '~':
     /* bchar -> achar -> uchar -> sub-delims-sh */
     case '!': case '$': case '\'': case '(': case ')': case '*':