Optimize isdigit, isgraph, islower, isupper

Verified to work

Author: myclevorname

src/libc/isdigit.src

@@ -8,7 +8,6 @@ _isdigit:
 	push	de
 	ld	a,l
 	sub	a,48
-	add	a,-10
+	sub	a,10
 	sbc	hl,hl
-	inc	hl
 	ret

src/libc/isgraph.src

@@ -8,7 +8,6 @@ _isgraph:
 	push	de
 	ld	a,l
 	sub	a,33
-	add	a,-94
+	sub	a,94
 	sbc	hl,hl
-	inc	hl
 	ret

src/libc/islower.src

@@ -8,7 +8,6 @@ _islower:
 	push	de
 	ld	a,l
 	sub	a,97
-	add	a,-26
+	sub	a,26
 	sbc	hl,hl
-	inc	hl
 	ret

src/libc/isupper.src

@@ -8,7 +8,6 @@ _isupper:
 	push	de
 	ld	a,l
 	sub	a,65
-	add	a,-26
+	sub	a,26
 	sbc	hl,hl
-	inc	hl
 	ret