Optimise build times by specifying only the generators we use in R.swift

[eviltofu]

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From the R.swift documentation By default, R.swift runs all generators, for images, nibs, strings and many more. In some situations you may not want to generate R structs for all these types. You can choose to run only certain generators by adding a flag like this: --generators image,string to the call to the Build Phase

These are the available generators:

image
string
color
file
font
nib
segue
storyboard
reuseIdentifier
entitlements
info
id

I believe we only use image, string, color, file, font. We should optimise this in order to reduce build times.