html/template: wraps package template instead of exposing func Escape

html/template: wraps package template instead of exposing func Escape

This does escaping on first execution.

template.go defines the same interface elements as package template.
It requires rather more duplication of code than I'd like, but I'm
not clear how to avoid that.

Maybe instead of

    mySet.ParseGlob(...)
    template.ParseSetGlob(...)
    mySet.ParseFiles(...)
    mySet.ParseTemplateFiles(...)
    template.ParseTemplateFiles(...)

we combine these into a fileset abstraction that can be wrapped

    var fileset template.FileSet
    fileset.Glob(...)  // Load a few files by glob
    fileset.Files(...)  // Load a few {{define}}d files
    fileset.TemplateFiles(...)  // Load a few files as template bodies
    fileset.Funcs(...)  // Make the givens func available to templates
    // Do the parsing.
    set, err := fileset.ParseSet()
    // or set, err := fileset.ParseInto(set)

or provide an interface that can receive filenames and functions and
parse messages:

    type Bundle interface {
      TemplateFile(string)
      File(string)
      Funcs(FuncMap)
    }

and define template.Parse* to handle the file-system stuff and send
messages to a bundle:

    func ParseFiles(b Bundle, filenames ...string)

R=r, r
CC=golang-dev
https://golang.org/cl/5270042