Introduce StringEncoder and StringDecoder types (#44)

Introduce StringEncoder and StringDecoder types

### Motivation

A sibling PR to #41, as part of moving from hand-crafted code that uses `LosslessStringConvertible` conformances on built-in and generated types, we need not just an URI coder, but also a raw string coder.

This will be used for encoding and decoding `text/plain` bodies, where only primitive types can be used, and are encoded as raw strings.

### Modifications

Introduced two types, `StringEncoder` and `StringDecoder`. It's very simple, doesn't support containers, only primitive types.

### Result

Once this lands, it'll allow us to get rid of our ugly marker protocols `_StringConvertible` and `_AutoLosslessStringConvertible` and always go through `Codable`. More on this in subsequent PRs integrating this logic.

### Test Plan

Added a rountrip unit test.


Reviewed by: glbrntt

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#44

Author: czechboy0