Move internal searchsorted* methods to _searchsorted*

[LilithHafner]

Technically, this is not a breaking change. Practically, we'll see. We might need to add in some new deprecated methods to ease the transition, or if folks have legitimate uses that warrant expanding the API to the 5-arg version we could add a new (safe) 5-arg version. See #51176 for motivation.

[LilithHafner]

Too breaking

[vtjnash]

Looks like a SparseArrays issue, since callers should be using view rather than this internal API. We can perhaps throw a boundscheck call in these though without much cost, and then defer to the _searchsorted version.

[LilithHafner]

I falsely assumed that views would have a performance overhead

julia> x = cumsum(rand(1000));

julia> @btime searchsortedfirst($x, 250, 400, 600, Base.Forward)
  14.612 ns (0 allocations: 0 bytes)
510

julia> @btime searchsortedfirst(view($x, 400:600), 250)
  14.612 ns (0 allocations: 0 bytes)
111

[vtjnash]

They should imply a boundscheck, I think, so not quite free. But we fixed the performance issue with using them here a couple Julia releases ago.