Transformations involving `StaticArray` is not type stable

[junyuan-chen]

One important benefit of using StaticArray is to avoid the memory allocation required for an Array. For now, there is type instability somewhere that results in allocations for type inference, which largely undermines the advantages of using StaticArray over Array.

using TransformVariables, StaticArrays, BenchmarkTools

tr = as(SVector{3, Float64})
a = rand(3)

julia> @btime transform($tr, $a)
  39.816 ns (2 allocations: 32 bytes)
3-element SVector{3, Float64} with indices SOneTo(3):
 0.8664582577624973
 0.27172807097850415
 0.5758008759582858

From @code_warntype, I see that there is type stability issue with transform_with.

[tpapp]

Thanks for the bug report and the MWE. Sorry for the delay, the fix was a bit tricky, will release a new version with it as soon as CI tests finish.