Drop support before go1.20 (#2022)

We still had old implementations of s2b and b2s that didn't require
unsafe.StringData and unsafe.SliceData that were added in go1.20. Since
we don't support go1.20 anymore we can drop these functions.

b2s.go

@@ -1,5 +1,3 @@
-//go:build go1.20
-
 package fasthttp
 
 import "unsafe"

b2s_old.go

@@ -1,14 +0,0 @@
-//go:build !go1.20
-
-package fasthttp
-
-import "unsafe"
-
-// b2s converts byte slice to a string without memory allocation.
-// See https://groups.google.com/forum/#!msg/Golang-Nuts/ENgbUzYvCuU/90yGx7GUAgAJ .
-//
-// Note it may break if string and/or slice header will change
-// in the future go versions.
-func b2s(b []byte) string {
-	return *(*string)(unsafe.Pointer(&b))
-}

fasthttpadaptor/b2s.go

@@ -1,5 +1,3 @@
-//go:build go1.20
-
 package fasthttpadaptor
 
 import "unsafe"

fasthttpadaptor/b2s_old.go

@@ -1,14 +0,0 @@
-//go:build !go1.20
-
-package fasthttpadaptor
-
-import "unsafe"
-
-// b2s converts byte slice to a string without memory allocation.
-// See https://groups.google.com/forum/#!msg/Golang-Nuts/ENgbUzYvCuU/90yGx7GUAgAJ .
-//
-// Note it may break if string and/or slice header will change
-// in the future go versions.
-func b2s(b []byte) string {
-	return *(*string)(unsafe.Pointer(&b))
-}

s2b.go

@@ -1,5 +1,3 @@
-//go:build go1.20
-
 package fasthttp
 
 import "unsafe"

s2b_old.go

@@ -1,21 +0,0 @@
-//go:build !go1.20
-
-package fasthttp
-
-import (
-	"reflect"
-	"unsafe"
-)
-
-// s2b converts string to a byte slice without memory allocation.
-//
-// Note it may break if string and/or slice header will change
-// in the future go versions.
-func s2b(s string) (b []byte) {
-	bh := (*reflect.SliceHeader)(unsafe.Pointer(&b))
-	sh := (*reflect.StringHeader)(unsafe.Pointer(&s))
-	bh.Data = sh.Data
-	bh.Cap = sh.Len
-	bh.Len = sh.Len
-	return b
-}