Macro-kit

New Features
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- Support function definition, if, for, and many other expressions.
- `Macro.callbacks` attribute for expression-added callbacks.
- `flags` keyword argument to specify what kind of expressions will be recorded.

Changes
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- `Macro` inherits `Expr` but head is fixed to `Head.block` to simplify module.

New Features
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- Support string parsing using `ast`.
- `optimize` method of `Macro` can remove unused returned variables. It makes macro looks better.

Changes
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- `Head` is different from that in `v0.1.0`, thus may be inconsistent.

This is the first release.