Make modbus library compatible with older versions of Toit. (#13)

Fixes #12.

examples/package.lock

@@ -7,5 +7,5 @@ packages:
       rs485: toit-rs485
   toit-rs485:
     url: github.com/toitware/toit-rs485
-    version: 1.0.0
-    hash: 2e8d0ae08c97a5dcba69b4e438408ef374c2c63f
+    version: 1.0.1
+    hash: 854d96a9d27b6ce7325a9698001087b292f4b4b5

package.lock

@@ -3,5 +3,5 @@ prefixes:
 packages:
   toit-rs485:
     url: github.com/toitware/toit-rs485
-    version: 1.0.0
-    hash: 2e8d0ae08c97a5dcba69b4e438408ef374c2c63f
+    version: 1.0.1
+    hash: 854d96a9d27b6ce7325a9698001087b292f4b4b5

package.yaml

@@ -1,4 +1,4 @@
 dependencies:
   rs485:
     url: github.com/toitware/toit-rs485
-    version: ^1.0.0
+    version: ^1.0.1

src/bus.toit

@@ -131,6 +131,22 @@ class Modbus:
   run:
     transactions_.run
 
+/**
+A signal synchronization primitive.
+
+This class is a partial copy of the 'Signal' class from the 'monitor' library. Older
+  versions of Toit don't have that class yet, and for backwards compatilibity we thus
+  copied it here.
+*/
+monitor Signal_:
+  /** Waits until the $condition is satisfied. */
+  wait [condition] -> none:
+    await: condition.call
+
+  /** Raises the signal, making waiters evaluate their condition. */
+  raise -> none:
+
+
 /**
 A transaction manager.
 
@@ -145,7 +161,7 @@ class TransactionManager_:
   max_concurrent_transactions_/int?
 
   transactions_/Map ::= {:}
-  signal_/monitor.Signal ::= monitor.Signal
+  signal_/Signal_ ::= Signal_
 
   run_task_ := null
   next_id_ := 0