Make use of TAG_PUBKEY constants in secp256k1_eckey_pubkey_parse

[Empact]

This results in more self-documenting code.

Constants defined here:

secp256k1/include/secp256k1.h

Lines 175 to 180 in 1e6f1f5

	/** Prefix byte used to tag various encoded curvepoints for specific purposes */
	#define SECP256K1_TAG_PUBKEY_EVEN 0x02
	#define SECP256K1_TAG_PUBKEY_ODD 0x03
	#define SECP256K1_TAG_PUBKEY_UNCOMPRESSED 0x04
	#define SECP256K1_TAG_PUBKEY_HYBRID_EVEN 0x06
	#define SECP256K1_TAG_PUBKEY_HYBRID_ODD 0x07

[gmaxwell]

Meh. I've never been a fan of replacing simple immediate (near-)single use immutable values with construction-opaque macros. But doing this here makes the code more consistent, so I will defer to the judgement that went into using EVEN and ODD in the surrounding code previously and ACK. I reserve the right to whine about the conversion of other simple constants elsewhere. :P