Rewrote README.

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This PR rewrites the README to include more up-to-date and cross-platform instructions/information. (See #609)

It also removes the sections about how to invoke the compiler, since this is going to be fundamentally changed by 0.2, when slicec becomes a standalone executable that outputs an encoded AST. I can add it back if you prefer... I wrote sections for:

• How to use the compiler (with the compile_from_options entry function).
• How to run the compiler directly (with cargo run --example parse ...).

But I ended up deleting them for the above-mentioned reason.

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For reference: