Follow-up from "Minor code optimisation #567"

[Vinícius Gajo]

Description:

@Thorium suggested:

Another modification to consider, would be adding [<Struct>] attribute to Giraffe's minor enum-types like HttpVerb, MatchMode, Precondition, CacheDirective and RangeBoundary. But if there are proper performance tests somewhere, could be first tested if they'd have any effect or not. But I guess that goes around questions like should some Option values changed to ValueOption values.

Reference to the PR: #567