Rework template handling for function and map lookups

This is a big commit, but it deletes lots of code and simplifies a lot.

* Resolving the template funcs at execution time means we don't have to create template clones per site
* Having a custom map resolver means that we can remove the AST lower case transformation for the special lower case Params map

Not only is the above easier to reason about, it's also faster, especially if you have more than one language, as in the benchmark below:

```
name                          old time/op    new time/op    delta
SiteNew/Deep_content_tree-16    53.7ms ± 0%    48.1ms ± 2%  -10.38%  (p=0.029 n=4+4)

name                          old alloc/op   new alloc/op   delta
SiteNew/Deep_content_tree-16    41.0MB ± 0%    36.8MB ± 0%  -10.26%  (p=0.029 n=4+4)

name                          old allocs/op  new allocs/op  delta
SiteNew/Deep_content_tree-16      481k ± 0%      410k ± 0%  -14.66%  (p=0.029 n=4+4)
```

This should be even better if you also have lots of templates.

Closes #6594

1 files changed, 13 insertions, 10 deletions

diff --git a/commands/server.go b/commands/server.go
index 709181507..7d884096c 100644
--- a/commands/server.go
+++ b/commands/server.go
@@ -16,6 +16,7 @@ package commands
 import (
 	"bytes"
 	"fmt"
+	"io"
 	"net"
 	"net/http"
 	"net/url"
@@ -33,7 +34,6 @@ import (
 	"github.com/pkg/errors"
 
 	"github.com/gohugoio/hugo/livereload"
-	"github.com/gohugoio/hugo/tpl"
 
 	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/helpers"
@@ -287,7 +287,7 @@ func getRootWatchDirsStr(baseDir string, watchDirs []string) string {
 type fileServer struct {
 	baseURLs      []string
 	roots         []string
-	errorTemplate tpl.Template
+	errorTemplate func(err interface{}) (io.Reader, error)
 	c             *commandeer
 	s             *serverCmd
 }
@@ -335,8 +335,7 @@ func (f *fileServer) createEndpoint(i int) (*http.ServeMux, string, string, erro
 				err := f.c.getErrorWithContext()
 				if err != nil {
 					w.WriteHeader(500)
-					var b bytes.Buffer
-					err := f.errorTemplate.Execute(&b, err)
+					r, err := f.errorTemplate(err)
 					if err != nil {
 						f.c.logger.ERROR.Println(err)
 					}
@@ -344,7 +343,7 @@ func (f *fileServer) createEndpoint(i int) (*http.ServeMux, string, string, erro
 					if !f.c.paused {
 						port = f.c.Cfg.GetInt("liveReloadPort")
 					}
-					fmt.Fprint(w, injectLiveReloadScript(&b, port))
+					fmt.Fprint(w, injectLiveReloadScript(r, port))
 
 					return
 				}
@@ -422,11 +421,15 @@ func (c *commandeer) serve(s *serverCmd) error {
 	}
 
 	srv := &fileServer{
-		baseURLs:      baseURLs,
-		roots:         roots,
-		c:             c,
-		s:             s,
-		errorTemplate: templ,
+		baseURLs: baseURLs,
+		roots:    roots,
+		c:        c,
+		s:        s,
+		errorTemplate: func(ctx interface{}) (io.Reader, error) {
+			b := &bytes.Buffer{}
+			err := c.hugo().Tmpl.Execute(templ, b, ctx)
+			return b, err
+		},
 	}
 
 	doLiveReload := !c.Cfg.GetBool("disableLiveReload")