[Fix] Re-introduce deprecated methods (#15)

[Fix] Re-introduce deprecated methods

### Motivation

Unfortunately, as part of landing apple/swift-openapi-generator#43, I made a mistake during backwards compatibility testing and forgot to add a few deprecated methods still used by generator 0.1.0.

That means that generator 0.1.0 doesn't work with runtime 0.1.1, currently.

This PR reintroduces them, so that generator 0.1.0 can work with a hotfix 0.1.2 again.

### Modifications

Re-added a few missing deprecated methods on Converter.

### Result

Someone using generator 0.1.0 with the latest runtime library (e.g. if they locked their generator version to 0.1.0 but didn't lock their runtime version) will have the generated code compiling again with runtime 0.1.2 (once this fix is landed).

### Test Plan

Checked locally by actually checking our the different tags of runtime and generator and built them together.


Reviewed by: simonjbeaumont

Builds:
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#15

Author: czechboy0