This commit introduces a standardized synthesis pipeline and restructures the codebase:
* Creates a new SynthesisPipeline class to define the default synthesis pipeline. This pipeline serves both circt-synth and is exposed through the C API for Python bindings.
* Added a dedicated Synthesis directory under `lib/` to house synthesis-related code. This architectural change is aimed to promote synthesis capabilities to a first-class component within CIRCT rather than limiting it to the circt-synth tool.
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