py: Add support for floats in mp_binary_{get,set}_val()

- This then provides support for floats in the struct package

tests/float/float_struct.py

@@ -0,0 +1,12 @@
+# test struct package with floats
+
+import struct
+
+i = 1. + 1/2
+# TODO: it looks like '=' format modifier is not yet supported
+# for fmt in ('f', 'd', '>f', '>d', '<f', '<d', '=f', '=d'):
+for fmt in ('f', 'd', '>f', '>d', '<f', '<d'):
+    x = struct.pack(fmt, i)
+    v = struct.unpack(fmt, x)[0]
+    print('%2s: %.17f - %s' % (fmt, v, (i == v) and 'passed' or 'failed'))
+