Rename *codec.Codec to *codec.LegacyAmino

[aaronc]

This type is now used only when we want to explicitly support legacy amino scenarios (queriers, REST). It is the type that should be used explicitly for these legacy scenarios. Eventually, even JSONMarshaler will need to require proto (see #6982). But for the cases we still really need amino we can use *codec.Codec. But let's give it a clear name, I propose *codec.LegacyAmino.

While we're doing this we should rename appropriate fields, specifically client.Context.Codec and simapp.Codec() should become *****.LegacyAmino.

[alexanderbez]

ACK

[dauTT]

Hi, can I work on this?

[aaronc]

Sure!