force specialization on arguments to core wrapper

[KristofferC]

Sometimes defining a function as f(a, b, c) = g(a, b, c) might mean that the specialization heuristic kicks in when the type of an argument is a Function or Type. This leads to issues like JuliaLang/julia#28087 and #71.

One possible way around this is to force specialization using where. This might make sense since presumably, we want this core wrapper to be "as invisible as possible".

Fixes #71.

julia> b1 = @benchmarkable rand($Float64); tune!(b1); run(b1)
BenchmarkTools.Trial:
  memory estimate:  0 bytes
  allocs estimate:  0
  --------------
  minimum time:     5.333 ns (0.00% GC)
  median time:      6.078 ns (0.00% GC)
  mean time:        6.379 ns (0.00% GC)
  maximum time:     23.800 ns (0.00% GC)
  --------------
  samples:          10000
  evals/sample:     1000

[jrevels]

Thanks!