added string.lowerchar() and string.upperchar()

compiler/res/prog8lib/string.p8

@@ -224,6 +224,29 @@ _done       rts
         }}
     }
 
+    asmsub lowerchar(ubyte char @A) -> ubyte @A {
+        %asm {{
+            and  #$7f
+            cmp  #97
+            bcc  +
+            cmp  #123
+            bcs  +
+            and  #%11011111
++           rts
+        }}
+    }
+
+    asmsub upperchar(ubyte char @A) -> ubyte @A {
+        %asm {{
+            cmp  #65
+            bcc  +
+            cmp  #91
+            bcs  +
+            ora  #%00100000
++           rts
+        }}
+    }
+
     sub startswith(str st, str prefix) -> bool {
         ubyte prefix_len = length(prefix)
         ubyte str_len = length(st)

compiler/res/prog8lib/virtual/string.p8

@@ -114,6 +114,18 @@ string {
         }
     }
 
+    sub lowerchar(ubyte char) -> ubyte {
+        if char >= 'A' and char <= 'Z'
+            char |= %00100000
+        return char
+    }
+
+    sub upperchar(ubyte char) -> ubyte {
+        if char >= 'a' and char <= 'z'
+            char &= %11011111
+        return char
+    }
+
     sub startswith(str st, str prefix) -> bool {
         ubyte prefix_len = length(prefix)
         ubyte str_len = length(st)

docs/source/libraries.rst

@@ -191,6 +191,12 @@ Provides string manipulation routines.
 ``upper(string)``
     Uppercases the petscii-string in place.
 
+``lowerchar(char)``
+    Returns lowercased character.
+
+``upperchar(char)``
+    Returns uppercased character.
+
 ``startswith(string, prefix) -> bool``
     Returns true if string starts with prefix, otherwise false