implement decoding charsets (other than ascii and utf-8) while reading textual message parts, and improve search

message.Part now has a ReaderUTF8OrBinary() along with the existing Reader().
the new function returns a reader of decoded content. we now use it in a few
places, including search. we only support the charsets in
golang.org/x/text/encoding/ianaindex.

search has also been changed to not read the entire message in memory. instead,
we make one 8k buffer for reading and search in that, and we keep the buffer
around for all messages. saves quite some allocations when searching large
mailboxes.

vendor/golang.org/x/text/encoding/ianaindex/ascii.go

@@ -0,0 +1,74 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ianaindex
+
+import (
+	"unicode"
+	"unicode/utf8"
+
+	"golang.org/x/text/encoding"
+	"golang.org/x/text/encoding/internal"
+	"golang.org/x/text/encoding/internal/identifier"
+	"golang.org/x/text/transform"
+)
+
+type asciiDecoder struct {
+	transform.NopResetter
+}
+
+func (d asciiDecoder) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err error) {
+	for _, c := range src {
+		if c > unicode.MaxASCII {
+			r := unicode.ReplacementChar
+			if nDst+utf8.RuneLen(r) > len(dst) {
+				err = transform.ErrShortDst
+				break
+			}
+			nDst += utf8.EncodeRune(dst[nDst:], r)
+			nSrc++
+			continue
+		}
+
+		if nDst >= len(dst) {
+			err = transform.ErrShortDst
+			break
+		}
+		dst[nDst] = c
+		nDst++
+		nSrc++
+	}
+	return nDst, nSrc, err
+}
+
+type asciiEncoder struct {
+	transform.NopResetter
+}
+
+func (d asciiEncoder) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err error) {
+	for _, c := range src {
+		if c > unicode.MaxASCII {
+			err = internal.RepertoireError(encoding.ASCIISub)
+			break
+		}
+
+		if nDst >= len(dst) {
+			err = transform.ErrShortDst
+			break
+		}
+		dst[nDst] = c
+		nDst++
+		nSrc++
+	}
+	return nDst, nSrc, err
+}
+
+var asciiEnc = &internal.Encoding{
+	Encoding: &internal.SimpleEncoding{
+		asciiDecoder{},
+		asciiEncoder{},
+	},
+	Name: "US-ASCII",
+	MIB:  identifier.ASCII,
+}