Consistently use {!r}, not '{!r}' in diagnostics.

This can cause confusion:
  * If the erroneous object is None, it is printed as 'None', which
    appears as a string (and could be the result of converting None
    to a string.)
  * If the erroneous object is a string, it is printed as ''<val>'',
    which is a rather strange combination of quotes.

nmigen/lib/cdc.py

@@ -9,7 +9,7 @@ __all__ += ["MultiReg"]
 
 def _check_stages(stages):
     if not isinstance(stages, int) or stages < 1:
-        raise TypeError("Synchronization stage count must be a positive integer, not '{!r}'"
+        raise TypeError("Synchronization stage count must be a positive integer, not {!r}"
                         .format(stages))
     if stages < 2:
         raise ValueError("Synchronization stage count may not safely be less than 2")

nmigen/lib/fifo.py

@@ -62,10 +62,10 @@ class FIFOInterface:
 
     def __init__(self, *, width, depth, fwft):
         if not isinstance(width, int) or width < 0:
-            raise TypeError("FIFO width must be a non-negative integer, not '{!r}'"
+            raise TypeError("FIFO width must be a non-negative integer, not {!r}"
                             .format(width))
         if not isinstance(depth, int) or depth < 0:
-            raise TypeError("FIFO depth must be a non-negative integer, not '{!r}'"
+            raise TypeError("FIFO depth must be a non-negative integer, not {!r}"
                             .format(depth))
         self.width = width
         self.depth = depth

nmigen/lib/io.py

@@ -13,13 +13,13 @@ def pin_layout(width, dir, xdr=0):
     See :class:`Pin` for details.
     """
     if not isinstance(width, int) or width < 1:
-        raise TypeError("Width must be a positive integer, not '{!r}'"
+        raise TypeError("Width must be a positive integer, not {!r}"
                         .format(width))
     if dir not in ("i", "o", "oe", "io"):
-        raise TypeError("Direction must be one of \"i\", \"o\", \"io\", or \"oe\", not '{!r}'"""
+        raise TypeError("Direction must be one of \"i\", \"o\", \"io\", or \"oe\", not {!r}"""
                         .format(dir))
     if not isinstance(xdr, int) or xdr < 0:
-        raise TypeError("Gearing ratio must be a non-negative integer, not '{!r}'"
+        raise TypeError("Gearing ratio must be a non-negative integer, not {!r}"
                         .format(xdr))
 
     fields = []