Limit ourselves to 8-bit branching

I'm planning on going back to 8-bit branching. 16-bit br cells incur
a non-negligible penalty and, while at first 64 words (128 bytes
forward or backward) seemed a bit limiting, I now don't see why one
would ever construct such a big branch. It would be un-forthy.

Also, I looked at using BC instead of IY to hold IP and the transition
would be a lot easier with 8-bit branching.

In this commit, all I do is add overflow checks in IF. The mechanic
below doesn't change. I'll give myself some time to think it over so
that I avoid yet another back and forth.

blk/004

@@ -1 +1,2 @@
 21 How blocks are organized    22 Addressed devices
+23 Branching

blk/023

@@ -0,0 +1,11 @@
+Branching
+
+Branching in Collapse OS is limited to 8-bit. This represents
+64 word references forward or backward. While this might seem
+a bit tight at first, having this limit saves us a non-
+negligible amount of resource usage.
+
+The reasoning behind this intentional limit is that huge
+branches are generally a indicator that a logic ought to be
+simplified. So here's one more constraint for you to help you
+towards simplicity.

blk/397

@@ -1,6 +1,7 @@
 ( Now we have "as late as possible" stuff )
+: _bchk DUP 0x7f + 0xff > IF LIT< br-ovfl (print) ABORT THEN ;
 : DO COMPILE 2>R H@ ; IMMEDIATE
-: LOOP COMPILE (loop) H@ - , ; IMMEDIATE
+: LOOP COMPILE (loop) H@ - _bchk , ; IMMEDIATE
 ( LEAVE is implemented in low xcomp )
 : LITN 32 , , ( 32 == NUMBER ) ;
 ( gets its name at the very end. can't comment afterwards )

blk/398

@@ -2,13 +2,12 @@
     COMPILE (?br) H@ 2 ALLOT ( br cell allot )
 ; IMMEDIATE
 : THEN ( a -- | a: br cell addr )
-    DUP H@ -^ SWAP ( a-H a ) !
+    DUP H@ -^ _bchk SWAP ( a-H a ) !
 ; IMMEDIATE
 : ELSE ( a1 -- a2 | a1: IF cell a2: ELSE cell )
     COMPILE (br)
     2 ALLOT
-    DUP H@ -^ SWAP  ( a-H a )
-    !
+    [COMPILE] THEN
     H@ 2- ( push a. -2 for allot offset )
 ; IMMEDIATE
 

blk/399

@@ -1,6 +1,6 @@
 : BEGIN H@ ; IMMEDIATE
-: AGAIN COMPILE (br) H@ - , ; IMMEDIATE
-: UNTIL COMPILE (?br) H@ - , ; IMMEDIATE
+: AGAIN COMPILE (br) H@ - _bchk , ; IMMEDIATE
+: UNTIL COMPILE (?br) H@ - _bchk , ; IMMEDIATE
 : [ INTERPRET ; IMMEDIATE
 : ] R> DROP ;
 : LIT< WORD 34 , SCPY 0 C, ; IMMEDIATE