Add comment support (#379)

* Add comment support

* Don't print empty comments

* Add nop instruction

* Add test and note

* Add FuncWriter trait

* Remove comment support

* Add write_preamble to FuncWriter

* Fix test

* Some changes

lib/codegen/meta-python/base/instructions.py

@@ -581,6 +581,13 @@ bconst = Instruction(
 # Generics.
 #
 
+nop = Instruction(
+        'nop', r"""
+        Just a dummy instruction
+
+        Note: this doesn't compile to a machine code nop
+        """)
+
 c = Operand('c', Testable, doc='Controlling value to test')
 x = Operand('x', Any, doc='Value to use when `c` is true')
 y = Operand('y', Any, doc='Value to use when `c` is false')