The `__future__` we relied on is now, where the 3 specific things are
all included [since Python 3.0](https://docs.python.org/3/library/__future__.html):
* absolute_import
* print_function
* unicode_literals
* division
These import statements are no-ops and are no longer necessary.
This change adds support for WASI in stdlib tests. Some tests that expect a crash to happen had to be disabled, since there's currently no way to observe such crash from a WASI host.
Previously, overflow and underflow both caused this to return `nil`, which causes several problems:
* It does not distinguish between a large but valid input and a malformed input. `Float("3.402824e+38")` is perfectly well-formed but returns nil
* It differs from how the compiler handles literals. As a result, `Float(3.402824e+38)` is very different from `Float("3.402824e+38")`
* It's inconsistent with Foundation Scanner()
* It's inconsistent with other programming languages
This is exactly the same as #25313
Fixes rdar://problem/36990878
This adjusts the standard library test suite to mostly pass on Windows.
The remaining failures are due to various cases:
- memory corruption (`_swift_stdlib_free` in swiftDemangle)
- heap corruption (canGrowUsingRealloc)
- withVAList failure (unresolved)
- unicode handling on the command line
