stdlib: remove embed wrapper

Embedding files using `//go:embed` and `embed` packages can not be supported in yaegi, so it is better to not generate the wrapper to embed and have a graceful error in case of usage of `embed.FS` in the interpreter.
Also update README about unsuported directives.

Fixes #1557.

README.md

@@ -172,6 +172,7 @@ Beside the known [bugs] which are supposed to be fixed in the short term, there
 
 - Assembly files (`.s`) are not supported.
 - Calling C code is not supported (no virtual "C" package).
+- Directives about the compiler, the linker, or embedding files are not supported.
 - Interfaces to be used from the pre-compiled code can not be added dynamically, as it is required to pre-compile interface wrappers.
 - Representation of types by `reflect` and printing values using %T may give different results between compiled mode and interpreted mode.
 - Interpreting computation intensive code is likely to remain significantly slower than in compiled mode.

stdlib/go1_19_embed.go

@@ -1,18 +0,0 @@
-// Code generated by 'yaegi extract embed'. DO NOT EDIT.
-
-//go:build go1.19 && !go1.20
-// +build go1.19,!go1.20
-
-package stdlib
-
-import (
-	"embed"
-	"reflect"
-)
-
-func init() {
-	Symbols["embed/embed"] = map[string]reflect.Value{
-		// type definitions
-		"FS": reflect.ValueOf((*embed.FS)(nil)),
-	}
-}

stdlib/go1_20_embed.go

@@ -1,18 +0,0 @@
-// Code generated by 'yaegi extract embed'. DO NOT EDIT.
-
-//go:build go1.20
-// +build go1.20
-
-package stdlib
-
-import (
-	"embed"
-	"reflect"
-)
-
-func init() {
-	Symbols["embed/embed"] = map[string]reflect.Value{
-		// type definitions
-		"FS": reflect.ValueOf((*embed.FS)(nil)),
-	}
-}

stdlib/stdlib.go

@@ -35,7 +35,7 @@ func init() {
 //go:generate ../internal/cmd/extract/extract crypto/subtle crypto/tls crypto/x509 crypto/x509/pkix
 //go:generate ../internal/cmd/extract/extract database/sql database/sql/driver
 //go:generate ../internal/cmd/extract/extract debug/buildinfo debug/dwarf debug/elf debug/gosym debug/macho debug/pe debug/plan9obj
-//go:generate ../internal/cmd/extract/extract embed encoding encoding/ascii85 encoding/asn1 encoding/base32
+//go:generate ../internal/cmd/extract/extract encoding encoding/ascii85 encoding/asn1 encoding/base32
 //go:generate ../internal/cmd/extract/extract encoding/base64 encoding/binary encoding/csv encoding/gob
 //go:generate ../internal/cmd/extract/extract encoding/hex encoding/json encoding/pem encoding/xml
 //go:generate ../internal/cmd/extract/extract errors expvar flag fmt