Mem opt part 3 b

[TeroJaasko]

This is the same base as https://github.com/ARMmbed/mbed-coap/pull/34, but with more optimizations and against current master, which had quite a few merge conflicts. I'll do squashing and move this to #34 after more tests. There are a lot of commits with XXX title, which exist just to ease git bisect in case of regression.

Base branch, ARMC5:

| mbed-coap/source/sn_coap_builder.o      |   2154(+0) |    0(+0) |      0(+0) |
| mbed-coap/source/sn_coap_header_check.o |    142(+0) |    0(+0) |      0(+0) |
| mbed-coap/source/sn_coap_parser.o       |   1710(+0) |    0(+0) |      0(+0) |
| mbed-coap/source/sn_coap_protocol.o     |   5730(+0) |    2(+0) |      0(+0) |
=9736

With this, ARMC5:

| mbed-coap/source/sn_coap_builder.o      |   1986(+0) |    0(+0) |      0(+0) |
| mbed-coap/source/sn_coap_header_check.o |    142(+0) |    0(+0) |      0(+0) |
| mbed-coap/source/sn_coap_parser.o       |   1704(+0) |    0(+0) |      0(+0) |
| mbed-coap/source/sn_coap_protocol.o     |   4868(+0) |    2(+0) |      0(+0) |
=8700

[anttiylitokola]

TestMds

[TeroJaasko]

TestMds

[TeroJaasko]

Closing this in favour of https://github.com/ARMmbed/mbed-coap/pull/87