Add httpx: low memory, non-allocating http implementation

[soypat]

Inspired entirely by the excellent fasthttp library: https://github.com/valyala/fasthttp/blob/master/header.go

Some notable changes to the base fasthttp library:

• Reduce stack usage by changing most internal []byte usage to string
• Trying to go for a single header type that condenses most HTTP logic, including response and requests
• Trying best to reduce the resulting size of program whenever possible
• Change some logic that allocates in TinyGo but does not in upstream Go

Still learning a lot about http while writing this, most likely will change.

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