Add a clearThrough instruction

This will let us fix lookahead assertions that have leftover save
points in the subpattern on success, and also allow us to implement
atomic groups.

Author: natecook1000

Sources/_StringProcessing/Engine/Instruction.swift

@@ -228,6 +228,13 @@ extension Instruction {
     /// Precondition: There is a save point to remove
     case clear
 
+    /// Remove save points up to and including the operand
+    ///
+    /// Operand: instruction address to look for
+    ///
+    /// Precondition: The operand is in the save point list
+    case clearThrough
+
     /// View the most recently saved point
     ///
     /// UNIMPLEMENTED

Sources/_StringProcessing/Engine/MEBuilder.swift

@@ -153,6 +153,10 @@ extension MEProgram.Builder {
   mutating func buildClear() {
     instructions.append(.init(.clear))
   }
+  mutating func buildClearThrough(_ t: AddressToken) {
+    instructions.append(.init(.clearThrough))
+    fixup(to: t)
+  }
   mutating func buildRestore() {
     instructions.append(.init(.restore))
   }
@@ -317,7 +321,7 @@ extension MEProgram.Builder {
       case .condBranchZeroElseDecrement:
         payload = .init(addr: addr, int: inst.payload.int)
 
-      case .branch, .save, .saveAddress, .call:
+      case .branch, .save, .saveAddress, .call, .clearThrough:
         payload = .init(addr: addr)
 
       case .splitSaving:

Sources/_StringProcessing/Engine/Processor.swift

@@ -250,6 +250,7 @@ extension Processor {
     }
     let (opcode, payload) = fetch().destructure
 
+  OpCodeSwitch:
     switch opcode {
     case .invalid:
       fatalError("Invalid program")
@@ -323,9 +324,21 @@ extension Processor {
       if let _ = savePoints.popLast() {
         controller.step()
       } else {
-        fatalError("TODO: What should we do here?")
+        // TODO: What should we do here?
+        fatalError("Invalid code: Tried to clear save points when empty")
       }
 
+    case .clearThrough:
+      let addr = payload.addr
+      while let sp = savePoints.popLast() {
+        if sp.pc == addr {
+          controller.step()
+          break OpCodeSwitch
+        }
+      }
+      // TODO: What should we do here?
+      fatalError("Invalid code: Tried to clear save points when empty")
+      
     case .peek:
       fatalError()