Check GenericIOBuffer data is contiguous

[nhz2]

Fix #54636 by throwing an error on construction of the GenericIOBuffer if its data isn't stored contiguously in memory.

[vtjnash]

That error may somewhat violate the expected contract. The unsafe_copyto should probably be replaced with just using normal reads from the source array, and only unsafe_load on the pointer argument

[nhz2]

There is already a fallback method for this in

julia/base/io.jl

Lines 334 to 339 in 4896473

	function unsafe_read(s::IO, p::Ptr{UInt8}, n::UInt)
	for i = 1:n
	unsafe_store!(p, read(s, UInt8)::UInt8, i)
	end
	nothing
	end

So it should be possible to just use that method if the GenericIOBuffer isn't wrapping a dense vector.