streaming multipart

[tanner0101]

Fixes #355

let parser = MultipartParser(boundary: "12345")

var parts: [MultipartPart] = []
var headers: [String: String] = [:]
var body: String = ""

parser.onHeader = { (field, value) in
    headers[field] = value
}
parser.onBody = { new in
    body += new.readString(length: new.readableBytes)!
}
parser.onPartComplete = {
    let part = MultipartPart(headers: headers, body: body)
    headers = [:]
    body = ""
    parts.append(part)
}

for chunk in data.chunked(by: i) {
    try parser.execute(.init(chunk))
}
print(parts)

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