Export method instance lookup functions

[vchuravy]

@ccall jl_method_lookup_by_tt(Tuple{typeof(+), Int, Int}::Any, Base.get_world_counter()::Csize_t)::Ref{Core.MethodInstance}

Ref JuliaGPU/GPUCompiler.jl#530 (comment)

[maleadt]

Performance of the slow path seems good enough for GPUCompiler?

julia> using BenchmarkTools

julia> @benchmark @ccall jl_method_lookup_by_tt(Tuple{typeof(identity), Nothing}::Any, Base.get_world_counter()::Csize_t)::Ref{Core.MethodInstance}
BenchmarkTools.Trial: 10000 samples with 991 evaluations.
 Range (min … max):  40.202 ns … 63.825 ns  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):     41.979 ns              ┊ GC (median):    0.00%
 Time  (mean ± σ):   41.988 ns ±  0.638 ns  ┊ GC (mean ± σ):  0.00% ± 0.00%

             ▁  ▂▂▁▂▃▁    ▂▅▇██▅▄▃▁                           ▂
  ▆▇▁▅▅▅▇▆▄▅▇█▆███████▇▆▇████████████▇▆▅▁▃▃▁▃▁▁▃▁▁▃▁▃▁▁▁▁▃▃▅▇ █
  40.2 ns      Histogram: log(frequency) by time        44 ns <

 Memory estimate: 0 bytes, allocs estimate: 0.

julia> @benchmark @ccall jl_method_lookup(Any[identity, nothing]::Ptr{Any}, 2::Csize_t, Base.get_world_counter()::Csize_t)::Ref{Core.MethodInstance}
BenchmarkTools.Trial: 10000 samples with 992 evaluations.
 Range (min … max):  38.800 ns …  15.855 μs  ┊ GC (min … max): 0.00% … 99.62%
 Time  (median):     41.764 ns               ┊ GC (median):    0.00%
 Time  (mean ± σ):   46.033 ns ± 172.107 ns  ┊ GC (mean ± σ):  5.05% ±  1.69%

         ▆▇▅         ▁█
  ▂▂▃▃▃▂▄███▇▄▂▂▂▁▂▂▃███▃▂▂▂▂▂▂▂▂▂▂▂▁▁▁▂▁▂▂▁▁▂▁▂▂▂▁▂▂▂▂▂▂▂▂▂▂▂ ▃
  38.8 ns         Histogram: frequency by time         56.8 ns <

 Memory estimate: 48 bytes, allocs estimate: 1.

julia> @benchmark @ccall jl_apply_generic(identity::Any, Any[nothing]::Ptr{Any}, 1::Csize_t)::Any
BenchmarkTools.Trial: 10000 samples with 995 evaluations.
 Range (min … max):  29.116 ns …  9.606 μs  ┊ GC (min … max): 0.00% … 99.47%
 Time  (median):     32.322 ns              ┊ GC (median):    0.00%
 Time  (mean ± σ):   32.950 ns ± 95.852 ns  ┊ GC (mean ± σ):  2.90% ±  0.99%

          ▅█       ▂▃▅▄▁                    ▂▃▃▃▆▄▄▃▅▂▂
  ▁▁▂▂▂▄▄▆██▆▅▃▃▄▄▆█████▇▄▃▃▂▂▂▁▁▁▁▂▃▃▆▇▆▆▆█████████████▇▄▂▂▂ ▄
  29.1 ns         Histogram: frequency by time        33.9 ns <

 Memory estimate: 32 bytes, allocs estimate: 1.