replace directive (#346)

Adds a `replace` directive that makes it easier to serialize foreign types.

Example usage with github.com/google/uuid

  ```go
  package main

  import "github.com/google/uuid"

  //go:generate msgp

  //msgp:replace uuid.UUID with:UUID
  type UUID [16]byte

  // Or like that
  //msgp:replace uuid.UUID with:[16]byte

  type User struct {
    ID uuid.UUID
  }
  ```

_generated/replace.go

@@ -0,0 +1,76 @@
+package _generated
+
+//go:generate msgp
+//msgp:replace Any with:any
+//msgp:replace MapString with:CompatibleMapString
+//msgp:replace MapAny with:map[string]any
+//msgp:replace SliceString with:[]string
+//msgp:replace SliceInt with:CompatibleSliceInt
+//msgp:replace Array8 with:CompatibleArray8
+//msgp:replace Array16 with:[16]byte
+//msgp:replace String with:string
+//msgp:replace Int with:CompatibleInt
+//msgp:replace Uint with:uint
+//msgp:replace Float32 with:CompatibleFloat32
+//msgp:replace Float64 with:CompatibleFloat64
+//msgp:replace Time with:time.Time
+//msgp:replace Duration with:time.Duration
+//msgp:replace StructA with:CompatibleStructA
+//msgp:replace StructB with:CompatibleStructB
+//msgp:replace StructC with:CompatibleStructC
+//msgp:replace StructD with:CompatibleStructD
+//msgp:replace StructI with:CompatibleStructI
+//msgp:replace StructS with:CompatibleStructS
+
+type (
+	CompatibleMapString map[string]string
+	CompatibleArray8    [8]byte
+	CompatibleInt       int
+	CompatibleFloat32   float32
+	CompatibleFloat64   float64
+	CompatibleSliceInt  []Int
+
+	// Doesn't work
+	// CompatibleTime time.Time
+
+	CompatibleStructA struct {
+		StructB StructB
+		Int     Int
+	}
+
+	CompatibleStructB struct {
+		StructC StructC
+		Any     Any
+		Array8  Array8
+	}
+
+	CompatibleStructC struct {
+		StructD StructD
+		Float64 Float32
+		Float32 Float64
+	}
+
+	CompatibleStructD struct {
+		Time      Time
+		Duration  Duration
+		MapString MapString
+	}
+
+	CompatibleStructI struct {
+		Int  *Int
+		Uint *Uint
+	}
+
+	CompatibleStructS struct {
+		Slice SliceInt
+	}
+
+	Dummy struct {
+		StructA StructA
+		StructI StructI
+		StructS StructS
+		Array16 Array16
+		Uint    Uint
+		String  String
+	}
+)

_generated/replace_ext.go

@@ -0,0 +1,54 @@
+package _generated
+
+import "time"
+
+// external types to test replace directive
+
+type (
+	MapString   map[string]string
+	MapAny      map[string]any
+	SliceString []String
+	SliceInt    []Int
+	Array8      [8]byte
+	Array16     [16]byte
+	Int         int
+	Uint        uint
+	String      string
+	Float32     float32
+	Float64     float64
+	Time        time.Time
+	Duration    time.Duration
+	Any         any
+
+	StructA struct {
+		StructB StructB
+		Int     Int
+	}
+
+	StructB struct {
+		StructC StructC
+		Any     Any
+		Array8  Array8
+	}
+
+	StructC struct {
+		StructD StructD
+		Float64 Float32
+		Float32 Float64
+	}
+
+	StructD struct {
+		Time      Time
+		Duration  Duration
+		MapString MapString
+	}
+
+	StructI struct {
+		Int  *Int
+		Uint *Uint
+	}
+
+	StructS struct {
+		Slice SliceInt
+	}
+)