Make tableswitch trap on out of bounds, rather than having a default.

AstSemantics.md

@@ -267,9 +267,9 @@ Other control constructs may yield values if their subexpressions yield values:
 
 ### Tableswitch
 
-A `tableswitch` consists of a zero-based array of targets, a *default* target, an index
+A `tableswitch` consists of a zero-based array of targets, an index
 operand, and a list of `case` nodes. Targets may be either labels or `case` nodes.
-A `tableswitch` jumps to the target indexed in the array or the default target if the index is out of bounds. 
+A `tableswitch` jumps to the target indexed in the array, or traps if the index is out of bounds.
 
 A `case` node consists of an expression and may be referenced multiple times
 by the parent `tableswitch`. Unless exited explicitly, control falls through a `case` 

Rationale.md

@@ -138,6 +138,14 @@ Also,
 [more expressive control flow constructs](FutureFeatures.md#more-expressive-control-flow)
 may be added in the future.
 
+The `tableswitch` instruction traps if the index is out of bounds rather than
+having a builtin "default" destination. Applications can perform their own
+range check if they require one. This does oblidge optimizing JITs to do more
+work to optimize away its own range checks, but the upside of making
+`tableswitch` be conceptually just a jump table, and not a jump table with an
+extra branch folded in, and it enables engines that have ways of doing the
+range check without using a branch to emit switches that have no branching at
+all in situations where the application itself doesn't need a default.
 
 ## Locals