Move toChars overrides to hdrgen.d

[dkorpel]

It's really confusing how both dmd.hdrgen calls into ASTNode.toChars, and toChars calls into dmd.hdrgen. It's also confusing how toChars is sometimes unused / incomplete / used for internal debugging, and other times expected to generate working D code / stringof / error messages. This moves towards letting ASTNode.toChars (or toString) call dmd.hdrgen, and differentiate uses with HdrGenState.

[dlang-bot]

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