Implement Double-Number and String word sets, fix memory panics

Double-Number (19 words): D+ D- DNEGATE DABS D2* D2/ D0= D0< D= D< DU<
  DMAX DMIN D>S M+ M*/ D. D.R 2ROT 2CONSTANT 2VARIABLE 2VALUE 2LITERAL
  Double-number literal parsing (tokens ending with '.')
String (5 words): COMPARE SEARCH /STRING BLANK -TRAILING SLITERAL
Fix all memory access panics with bounds checking throughout host functions.

8 word sets at 100%: Core, Core Ext, Exception, Double, String,
  Search-Order, Memory-Allocation, Programming-Tools

crates/core/src/ir.rs

@@ -93,7 +93,7 @@ pub enum IrOp {
         test: Vec<IrOp>,
         body: Vec<IrOp>,
     },
-    /// BEGIN test1 WHILE test2 WHILE body REPEAT after_repeat ELSE else_body THEN
+    /// BEGIN test1 WHILE test2 WHILE body REPEAT `after_repeat` ELSE `else_body` THEN
     ///
     /// Two nested WHILEs in a single BEGIN loop. When the first WHILE fails,
     /// control goes to `else_body`. When the second WHILE fails, control goes