FormData, URLSearchParams and other HTTP classes

[g105b]

These two classes are fundamental to basic HTTP usage, but specifically need to be implemented to allow PHP.Gt/Fetch to reach its next stable release.

phpgt/Fetch#99 relies on this issue.

Class list to implement:

• FormData
• URLSearchParams
• TypedArray
• DataView not supported for PHP (I think...)
• Blob
• FileUpload (or similarly named class)

[g105b]

Still need to:

• test real file uploads
• implement promise-based returns (that automatically resolve), like Blob::text() for example

[g105b]

The latest commit implements these classes, but the file upload functionality will be handled separately in #213