Export GetNextSize and implement ReadNext

It is useful to be able to efficiently copy objects without
decoding them.

My use case is filtering when I already know the indices of
the objects I want to keep, and for rewriting a dictionary
of objects as a column of objects.

This commit:

1. Exports `GetNextSize`.
2. Adds a method `ReadNext(p)` to `*Reader`.

I wasn't sure about exporting `GetNextSize`, but I can't see the
harm in it, and it may be useful for finer-grained control in
the copying.

I also experimented with a `NextReader() io.Reader` method which
returned `io.LimitReader(m.R, GetNextSize()), but for my use case
this was twice as slow and also did not handle nested objects.
In principle that might be useful for very large simple objects,
but is not included in this PR.

msgp/read.go

@@ -1,6 +1,7 @@
 package msgp
 
 import (
+	"fmt"
 	"io"
 	"math"
 	"sync"
@@ -146,6 +147,26 @@ func (m *Reader) Read(p []byte) (int, error) {
 	return m.R.Read(p)
 }
 
+// ReadNext reads the raw bytes for the next object on the wire into p.
+// If p is not large enough, an error is returned. See GetNextSize.
+func (m *Reader) ReadNext(p []byte) (int, error) {
+	sz, _, err := GetNextSize(m.R)
+	if err != nil {
+		return 0, err
+	}
+	if uintptr(len(p)) < sz {
+		return 0, fmt.Errorf("p not big enough (%d < %d)", len(p), sz)
+	}
+	n, err := m.R.ReadFull(p[:sz])
+	if err != nil {
+		return 0, err
+	}
+	if n != int(sz) {
+		return 0, fmt.Errorf("wrong # bytes read (%d != %d)", n, int64(sz))
+	}
+	return n, err
+}
+
 // ReadFull implements `io.ReadFull`
 func (m *Reader) ReadFull(p []byte) (int, error) {
 	return m.R.ReadFull(p)
@@ -194,12 +215,13 @@ func (m *Reader) IsNil() bool {
 	return err == nil && p[0] == mnil
 }
 
+// GetNextSize returns the size of the next object on the wire.
 // returns (obj size, obj elements, error)
 // only maps and arrays have non-zero obj elements
 //
 // use uintptr b/c it's guaranteed to be large enough
 // to hold whatever we can fit in memory.
-func getNextSize(r *fwd.Reader) (uintptr, uintptr, error) {
+func GetNextSize(r *fwd.Reader) (uintptr, uintptr, error) {
 	b, err := r.Peek(1)
 	if err != nil {
 		return 0, 0, err
@@ -261,7 +283,7 @@ func (m *Reader) Skip() error {
 			return err
 		}
 	} else {
-		v, o, err = getNextSize(m.R)
+		v, o, err = GetNextSize(m.R)
 		if err != nil {
 			return err
 		}

msgp/read_bytes.go

@@ -117,7 +117,7 @@ func (r Raw) Msgsize() int {
 }
 
 func appendNext(f *Reader, d *[]byte) error {
-	amt, o, err := getNextSize(f.R)
+	amt, o, err := GetNextSize(f.R)
 	if err != nil {
 		return err
 	}