fix #28481, faster reading of primitive types from IOStream and IOBuffer

[JeffBezanson]

This adds a fast implementation of reading Base numeric types from an IOBuffer, and extends the existing method for IOStream to floats.

fix #28481

[andyferris]

This is really nice to see. Any impressive benchmarks?

Also, is there some way to make this generic for bitstypes?

[JeffBezanson]

Here's a test case with a random ~22MB file. rd is a function that reads the whole file by repeatedly reading the given type. f is the file as an IOStream, b is the same data copied to an IOBuffer.
Before:

julia> @time rd(f, Float32)
  0.114615 seconds (5.72 M allocations: 87.302 MiB, 4.46% gc time)

julia> @time rd(b, Float32)
  0.079160 seconds (5.72 M allocations: 87.302 MiB, 3.16% gc time)

After:

julia> @time rd(f, Float32)
  0.073971 seconds (4 allocations: 160 bytes)

julia> @time rd(b, Float32)
  0.014330 seconds (4 allocations: 160 bytes)

[vtjnash]

insert from.ptr += avail? Although I don't know how strict we are about this consuming data before errors.