Use impl Trait return type for next_string_reader and string_value_writer

[Marcono1234]

Problem solved by the enhancement

Currently JsonReader::next_string_reader and JsonWriter::string_value_writer have Box<...> as return type because:

• They don't want to expose the internal implementation structs as public API
• Custom JsonReader and JsonWriter implementation should be able to implement those methods

However, this usage of Box means that heap allocations are performed.

Enhancement description

Use the impl Trait in return positions feature (RFC 3425) once it is available in stable Rust.

This allows avoiding usage of Box while still addressing the concerns listed above for why Box is currently used.

A proof-of-concept implementation of this is in https://github.com/Marcono1234/struson/tree/impl-trait-return (has to be slightly adjusted once this Rust feature is part of the stable toolchain).

Alternatives / workarounds

• Define public structs for string value reader and writer and declared them as return type;
  that would however prevent custom JsonReader and JsonWriter implementations from implementing the methods returning these value readers and writers

[Marcono1234]

The Rust blog post discourages using -> impl Trait in public API. However, since the current API returns Box<dyn Read + '_> respectively Box<dyn StringValueWriter + '_>, using -> impl Trait seems equally good / bad, except that as mentioned above it avoids the heap allocations from Box.

So using -> impl Trait should probably be fine.