py/objexcept: Allow compression of exception message text.

The decompression of error-strings is only done if the string is accessed
via printing or via er.args.  Tests are added for this feature to ensure
the decompression works.

tests/micropython/heapalloc_exc_compressed.py

@@ -0,0 +1,48 @@
+import micropython
+
+# Tests both code paths for built-in exception raising.
+# mp_obj_new_exception_msg_varg (exception requires decompression at raise-time to format)
+# mp_obj_new_exception_msg (decompression can be deferred)
+
+# NameError uses mp_obj_new_exception_msg_varg for NameError("name '%q' isn't defined")
+# set.pop uses mp_obj_new_exception_msg for KeyError("pop from an empty set")
+
+# Tests that deferred decompression works both via print(e) and accessing the message directly via e.args.
+
+a = set()
+
+# First test the regular case (can use heap for allocating the decompression buffer).
+try:
+    name()
+except NameError as e:
+    print(type(e).__name__, e)
+
+try:
+    a.pop()
+except KeyError as e:
+    print(type(e).__name__, e)
+
+try:
+    name()
+except NameError as e:
+    print(e.args[0])
+
+try:
+    a.pop()
+except KeyError as e:
+    print(e.args[0])
+
+# Then test that it still works when the heap is locked (i.e. in ISR context).
+micropython.heap_lock()
+
+try:
+    name()
+except NameError as e:
+    print(type(e).__name__)
+
+try:
+    a.pop()
+except KeyError as e:
+    print(type(e).__name__)
+
+micropython.heap_unlock()

tests/micropython/heapalloc_exc_compressed.py.exp

@@ -0,0 +1,6 @@
+NameError name 'name' isn't defined
+KeyError pop from an empty set
+name 'name' isn't defined
+pop from an empty set
+NameError
+KeyError

tests/micropython/heapalloc_exc_compressed_emg_exc.py

@@ -0,0 +1,41 @@
+import micropython
+
+# Does the full test from heapalloc_exc_compressed.py but while the heap is
+# locked (this can only work when the emergency exception buf is enabled).
+
+# Some ports need to allocate heap for the emgergency exception buffer.
+try:
+    micropython.alloc_emergency_exception_buf(256)
+except AttributeError:
+    pass
+
+a = set()
+
+
+def test():
+    micropython.heap_lock()
+
+    try:
+        name()
+    except NameError as e:
+        print(type(e).__name__, e)
+
+    try:
+        a.pop()
+    except KeyError as e:
+        print(type(e).__name__, e)
+
+    try:
+        name()
+    except NameError as e:
+        print(e.args[0])
+
+    try:
+        a.pop()
+    except KeyError as e:
+        print(e.args[0])
+
+    micropython.heap_unlock()
+
+
+test()

tests/micropython/heapalloc_exc_compressed_emg_exc.py.exp

@@ -0,0 +1,4 @@
+NameError name 'name' isn't defined
+KeyError pop from an empty set
+name 'name' isn't defined
+pop from an empty set