py: Big improvements to inline assembler.

Improved the Thumb assembler back end.  Added many more Thumb
instructions to the inline assembler.  Improved parsing of assembler
instructions and arguments.  Assembler functions can now be passed the
address of any object that supports the buffer protocol (to get the
address of the buffer).  Added an example of how to sum numbers from
an array in assembler.

examples/asmsum.py

@@ -0,0 +1,57 @@
+@micropython.asm_thumb
+def asm_sum_words(r0, r1):
+
+    # r0 = len
+    # r1 = ptr
+    # r2 = sum
+    # r3 = dummy
+    mov(r2, 0)
+
+    b(loop_entry)
+
+    label(loop1)
+    ldr(r3, [r1, 0])
+    add(r2, r2, r3)
+
+    add(r1, r1, 4)
+    sub(r0, r0, 1)
+
+    label(loop_entry)
+    cmp(r0, 0)
+    bgt(loop1)
+
+    mov(r0, r2)
+
+@micropython.asm_thumb
+def asm_sum_bytes(r0, r1):
+
+    # r0 = len
+    # r1 = ptr
+    # r2 = sum
+    # r3 = dummy
+    mov(r2, 0)
+
+    b(loop_entry)
+
+    label(loop1)
+    ldrb(r3, [r1, 0])
+    add(r2, r2, r3)
+
+    add(r1, r1, 1)
+    sub(r0, r0, 1)
+
+    label(loop_entry)
+    cmp(r0, 0)
+    bgt(loop1)
+
+    mov(r0, r2)
+
+import array
+
+b = array.array('l', (100, 200, 300, 400))
+n = asm_sum_words(len(b), b)
+print(b, n)
+
+b = array.array('b', (10, 20, 30, 40, 50, 60, 70, 80))
+n = asm_sum_bytes(len(b), b)
+print(b, n)