feat: Add directive for "encoding" binary interfaces

[klauspost]

Allow specifying directives for (external) types that support BinaryMarshaler + BinaryUnmarshaler and optionally BinaryAppender.

• //msgp:binmarshal pkg.Type pkg.Type2 will use BinaryMarshaler/BinaryUnmarshaler.
• //msgp:binappend pkg.Type pkg.Type2 will use BinaryAppender/BinaryUnmarshaler.

Serialized data will be added an bin data, no special header.

I guess we could also add the text interfaces. But the user would probably need to specify the storage type...

• //msgp:textmarshal pkg.Type pkg.Type2 will use TextMarshaler/TextUnmarshaler; data saved as bin (default)
• //msgp:textappend pkg.Type pkg.Type2 will use TextAppender/TextUnmarshaler; data saved as bin (default)
• //msgp:textmarshal as:string pkg.Type pkg.Type2 will use TextMarshaler/TextUnmarshaler; data saved as string.
• //msgp:textappend as:string pkg.Type pkg.Type2 will use TextAppender/TextUnmarshaler; data saved as string.

Missing anything, suggestions?