Deprecate unused Encoder trait methods

[bluss]

A gang of trait methods are currently unused and should probably be deprecated and removed.

Unused methods are at least:

emit_enum_struct_variant
emit_enum_struct_variant_field
emit_tuple_arg
emit_tuple_struct
emit_tuple_struct_arg

[omasanori]

What do you mean by currently unused? Are they unused in encoders bundled in libserialize, or existing libraries (on GitHub?) which implement Encoder?

[bluss]

• emit_tuple_arg is not used by the tuple implementations of Encodable, they instead use emit_seq_elt, so this method is redundant and unused.
• emit_enum_struct_variant and emit_enum_struct_variant_field and emit_tuple_struct and emit_tuple_struct_arg aren't used by automatic deriving of Encodable -- struct variants in enum instead use emit_enum_variant. Tuple structs instead use emit_struct. In this sense the methods are redundant and unused.

[bluss]

IMO they need to either be used by libstd and similar places (libsyntax) or deprecated. Using emit_tuple_arg is a good idea.

[steveklabnik]

Encode was moved out of tree, so if this is still an issue, please persue it over at https://github.com/rust-lang/rustc-serialize