Allow generating source maps.

[gdotdesign]

This PR builds on and supersedes #356 (Mainly the VLQ implementation is brought over).

• SPEC: https://tc39.es/source-map/
• Testing: https://evanw.github.io/source-map-visualization/ https://sokra.github.io/source-map-visualization/
• I've tested this with mint-website and mint-ui-website and it seems to be working.
• It's slow at this time (~1s from a ~2.2s build).

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Developer Notes:

• Because of the new compiler architecture it was relatively easy to implement.
• Changes in specs are because the order of resolving entities changed.

TODO:

• Mappings and sources
• Wiring to commands (--generate-source-maps flag)
• Symbol names for the debugger
• Cleanup pass

The reason for the slowness is Ast::Node#location calculation from offset to line/column, which needs to be calculated when parsing so it will be in a separate PR.