[ ux ] Don't export name prf, as it's widely used and warned about

[buzden]

Compiler produces a warning when some top-level definition is shadowed. All record fields produce top-level getter functions with the same name. Name prf is very commonly used as small proof function arguments. Thus, everyone using this library gets lots of shadowing warnings because Layout exports a field named prf.

I propose to rename this field.

[Z-snails]

Thanks