Check BURNTSUSHI_TOML_110 in the parser

Checking it on import means applications won't be to set it with SetEnv
in the application itself.

Fixes #394

decode_test.go

@@ -18,8 +18,8 @@ import (
 )
 
 func WithTomlNext(f func()) {
-	tomlNext = true
-	defer func() { tomlNext = false }()
+	os.Setenv("BURNTSUSHI_TOML_110", "")
+	defer func() { os.Unsetenv("BURNTSUSHI_TOML_110") }()
 	f()
 }
 

parse.go

@@ -11,10 +11,7 @@ import (
 	"github.com/BurntSushi/toml/internal"
 )
 
-var tomlNext = func() bool {
-	_, ok := os.LookupEnv("BURNTSUSHI_TOML_110")
-	return ok
-}()
+var tomlNext bool
 
 type parser struct {
 	lx         *lexer
@@ -35,6 +32,9 @@ type keyInfo struct {
 }
 
 func parse(data string) (p *parser, err error) {
+	_, ok := os.LookupEnv("BURNTSUSHI_TOML_110")
+	tomlNext = ok
+
 	defer func() {
 		if r := recover(); r != nil {
 			if pErr, ok := r.(ParseError); ok {