Macro to unroll fast printing

[GabrielKS]

Currently we have lots of manually unrolled blocks like this:

PowerFlows.jl/src/psse_export.jl

Lines 311 to 316 in 17b0423

	fastprintdelim(io, IC)
	fastprintdelim(io, SBASE)
	fastprintdelim(io, REV)
	fastprintdelim(io, XFRRAT)
	fastprintdelim(io, NXFRAT)
	fastprintln(io, BASFRQ)

because things like this were thought to be insufficiently performant in a tight loop:

PowerFlows.jl/src/psse_export.jl

Line 354 in f00e73b

joinln(io, (IC, SBASE, REV, XFRRAT, NXFRAT, BASFRQ)) # Tuple to avoid type promotion

. The current situation is kind of messy, though. Assuming that the unrolling is actually useful for performance, I'd like to create a macro that does the same thing.

See 465e92f where the manual unrolling was implemented.