Remove microcycle unit

Introducing it complicates things and has no advantage with good rational arithmetic support.

artiq/__init__.py

@@ -1,3 +1,3 @@
 from artiq.language.core import *
-from artiq.language.units import microcycle, ps, ns, us, ms, s
+from artiq.language.units import ps, ns, us, ms, s
 from artiq.language.units import Hz, kHz, MHz, GHz

artiq/language/core.py

@@ -306,7 +306,7 @@ def delay(duration):
 
 
 def now():
-    """Retrieves the current RTIO time, in microcycles.
+    """Retrieves the current RTIO time, in seconds.
 
     """
     return _time_manager.get_time()

artiq/language/units.py

@@ -157,6 +157,3 @@ def _register_unit(name, prefixes):
 
 _register_unit("s", "pnum_")
 _register_unit("Hz", "_kMG")
-
-microcycle_unit = Unit("microcycle")
-microcycle = Quantity(1, microcycle_unit)

artiq/transforms/inline.py

@@ -49,7 +49,7 @@ class _ReferenceManager:
             self.use_count[kg] = 1
         for name in ("int", "round", "int64", "round64", "float", "array",
                      "range", "Fraction", "Quantity", "EncodedException",
-                     "s_unit", "Hz_unit", "microcycle_unit"):
+                     "s_unit", "Hz_unit"):
             self.use_count[name] = 1
 
     def new_name(self, base_name):

artiq/transforms/interleave.py

@@ -7,7 +7,7 @@ from artiq.transforms.tools import *
 # -1 statement duration could not be pre-determined
 #  0 statement has no effect on timeline
 # >0 statement is a static delay that advances the timeline
-#     by the given amount (in microcycles)
+#     by the given amount
 def _get_duration(stmt):
     if isinstance(stmt, (ast.Expr, ast.Assign)):
         return _get_duration(stmt.value)

artiq/transforms/lower_units.py

@@ -24,9 +24,6 @@ class _UnitsLowerer(ast.NodeTransformer):
             self.in_core_time = old_in_core_time
         elif fn == "Quantity":
             if self.in_core_time:
-                if node.args[1].id == "microcycle_units":
-                    node = node.args[0]
-                else:
                 node = ast.copy_location(
                     ast.BinOp(left=node.args[0],
                               op=ast.Div(),