Сompatibility with msgppack-php

[darvik80]

1. Changes in read*.go compatibility with https://github.com/msgpack/msgpack-php
2. Changes in gen/elem.go && gen/spec.go fix it:

//go:generate msgp
package main

type SomeStruct map[string]string

type SomeMegaStruct struct {
    Struct1 *SomeStruct `msg:"struct-1"`
    Struct2 *SomeStruct `msg:"struct-2"`
}

[philhofer]

The best way to read numbers that come from languages that do not draw distinctions between the signed and unsigned integers (or floating-point numbers, for that matter) is to use msgp.Number, which will happily decode signed and unsigned integers and single- and double-precision floats.

[client9]

see also #134

[shabbyrobe]

I think this is covered by #215 too?