libstd: De-mut arena
librustc: Update the serializer to work properly with INHTWAMA, removing mutable fields in the process
auto merge of #6201 : pcwalton/rust/inhtwama-serializer, r=graydon

This PR removes mutable fields from the serializer and makes the encoder and decoder use INHTWAMA properly (i.e. `&mut self`).

r? @graydon