FoundationEssentials: support mmap'ed reading on Windows

[compnerd]

This adjusts the data reading path for supporting Windows which does not have the mmap function but does provide memory-mapped IO. The difference in the API is sufficient to create a separate path for the wrapping function rather than trying to have a single shared implementation that simply uses different APIs for the system integration.

[compnerd]

Note that this doesn't currently work as I don't know how to handle the errors. The error information is not really an errno (POSIX error code) but rather a Win32 Error (GetLastError). The build failure on Windows for this file is now just the error reporting.

[jmschonfeld]

Note that this doesn't currently work as I don't know how to handle the errors

CocoaError.errorWithFilePath with an errno just creates a CocoaError by mapping the errno to an applicable CocoaError.Code and then creates a CocoaError with that code and a user info dictionary with the underlying POSIXError. Would we be able to create another overload of errorWithFilePath but instead of an errno: parameter perhaps with a win32Error: parameter that similarly maps error codes to CocoaError.Code and then creates a user info dictionary with just the underlying code instead of using a POSIXError?

[compnerd]

Thanks @jmschonfeld that sounds like it should be possible to do!

[compnerd]

I've created #528 with the new error mapping that can hopefully be used to build this out.

[compnerd]

@swift-ci please test

[compnerd]

@swift-ci please test