use lookupMeta* functions wherever possible

[mb21]

depends on jgm/pandoc-types#51

[mb21]

Okay, this is now changed to include the new functions in Text.Pandoc.Writers.Shared instead of pandoc-types.

[mb21]

@jgm, due to d7263a7, there was a conflict again. You added metaValueToString there, analogous to the metaValueToInlines functions which I removed in favour of the lookupMetaInlines. (I don't really feel strongly either way, but most usage makes this variant simpler.)

So I rebased once more. Would be really good to merge this, since otherwise people will continue adding stuff that transforms the metadata inconsistently to string/inline/block/bool, etc. like just happened now.

[jgm]

@mb21, sorry about that. I'll merge!

[mb21]

Thanks!