g-l: Use language-specific heredoc delimiters for C code

[issyl0]

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Carlo had the great idea to use language-specific heredoc delimiters rather than EOS. There's a RuboCop for this, but before we enable it for "EOS" we have to get rid of (the majority of?) the uses of EOS for non-caveats.

Here's a first pass at g* to l* formulae with C code inside heredocs because that was easily greppable.