…tyle (#127)

### Motivation

As part of apple/swift-openapi-generator#259,
adding deepObject parameter style support, the initial PR wasn't
complete.

And once we dug more into it, turns out the original implementation of
the URIDecoder/URIParser didn't really lend themselves well for handling
deepObject, and the recent additions of supporting arrays within
dictionaries (#120)
further confused the implementation.

### Modifications

Refactored URIParser/URIDecoder with a clearer understanding of the
current requirements. It's now much easier to follow and embraces the
fact that each of the 7 variants of URI coding we support (form
exploded, form unexploded, simple exploded, simple unexploded, form data
exploded, form data unexploded, and now deepObject exploded) are
similar, but still different in subtle ways.

This new implementation doesn't try as hard to share code between the
implementations, so might at first sight appear to duplicate code. The
original implementation had many methods with many configuration
parameters and utility methods with a high cyclomatic complexity, which
made it very hard to reason about. We did away with that.

While there, I also made some minor improvements to the serialization
path, which allows cleaner round-tripping tests.

### Result

A more maintainable and more correct URI decoder/parser implementation.

### Test Plan

Added many more unit tests that test the full matrix of supported styles
and inputs.

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Co-authored-by: Si Beaumont <simonjbeaumont@gmail.com>