[Agda] Optimised handling of records

[Nils Anders Danielsson]

Hi,

Ulf and I have optimised the handling of records. If you have
encountered performance problems when, say, formalising category theory,
you may want to try again. For one example we measured a speedup of ∼5,
while the standard library type checks ∼20% faster (on my machine).

Please let us know if you still have problems working with record
hierarchies.

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/NAD