specialize on mapfoldl(::Type, ...) (#39019)
Before:
```julia
julia> using BenchmarkTools
julia> b = rand(Bool, 10000);
julia> @btime mapfoldl(Int, +, b; init=0)
1.136 ms (9025 allocations: 141.02 KiB)
5057
```
After:
```julia
julia> @btime mapfoldl(Int, +, b; init=0)
1.094 μs (1 allocation: 16 bytes)
5057
```
