MCS: add Channel Coding Command encoder

Add function to encode MCS value as proper EDGE or GPRS Channel Coding
value according to 3GPP TS 44.060 and corresponding helpers.

Use it for everything except IA Rest Octet encoding which is done in a
follow-up patches to make sure that we distinguish between
encoding-related changes to test output and unrelated changes.

Change-Id: I127fb29f5aaf77a7f6c4c565dfeb3b711af9845d
diff --git a/src/gprs_coding_scheme.h b/src/gprs_coding_scheme.h
index 728ffd6..d5604fb 100644
--- a/src/gprs_coding_scheme.h
+++ b/src/gprs_coding_scheme.h
@@ -82,6 +82,7 @@
 	enum CodingScheme m_scheme;
 };
 
+// FIXME: remove once < comparison operator below is no longer necessary
 inline uint8_t GprsCodingScheme::to_num() const
 {
 	if (mcs_is_gprs(m_scheme))