Support JSONSerialization of more number types

[seabaylea]

Currently the JSON Serializer only handles the "common" numbers types: Int, Double and NSNumber.

Trying to serialise the other number types: UInt, Int8, UInt8, etc as well as Float results in an Error.

This PR adds support for the wider set of standard types.

[phausler]

This is incompatible with the Darwin version:

let json = ["foo" : UInt8(4)]
let data = try! JSONSerialization.data(withJSONObject: json, options: [])

print("\(String(data: data, encoding: .utf8)!)")

fails with the exception:

2016-09-21 12:34:16.233614 JSONTest[50614:1325824] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Invalid type in JSON write (_SwiftValue)'
*** First throw call stack:
(
    0   CoreFoundation                      0x00007fff81f4442b __exceptionPreprocess + 171
    1   libobjc.A.dylib                     0x00007fff9689ca2a objc_exception_throw + 48
    2   CoreFoundation                      0x00007fff81fc2c05 +[NSException raise:format:] + 197
    3   Foundation                          0x00007fff838e973a _writeJSONValue + 698
    4   Foundation                          0x00007fff83946636 ___writeJSONObject_block_invoke + 226
    5   CoreFoundation                      0x00007fff81eb863e __NSDictionaryEnumerate + 862
    6   Foundation                          0x00007fff8394642a _writeJSONObject + 430
    7   Foundation                          0x00007fff838e965f _writeJSONValue + 479
    8   Foundation                          0x00007fff838e942c -[_NSJSONWriter dataWithRootObject:options:error:] + 124
    9   Foundation                          0x00
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb) 

[parkera]

Why does Int8 bridge to _SwiftValue instead of NSNumber in the first place? I thought this was something they were looking into fixing.

[phausler]

The changes for numeric bridging have not yet landed, but to replicate that behavior then we should funnel the top level input to _SwiftValue.store() and traverse references instead of structures.

[seabaylea]

I've updated to use _SwiftValue.store(obj) for the various number types, which matches the behaviour on Darwin.

[phausler]

@swift-ci please test

[parkera]

@phausler you ok with this? should we merge now?

[phausler]

Yea, this seems like a good first step to gaining more pairity