First cut of deprecating old helper syntax

with back-compat.

The “old-style” syntax is `Template.foo.bar = …` instead of `Template.foo.helpers({bar: …})`.

Properties of back-compat to test:  You can replace an old-style helper (overwrite it).  You can delete an old-style helper.  We don’t have to support these capabilities, but we used them both in our test code, so apps probably do it too.

We print a deprecation once per helper.  This leads to a lot of warnings!  It’s helpful for porting, though developers who for some reason don’t want to port yet may want a way to disable them.

`spacebars-tests` pass.
Add tests for old-style helper back-compat
Port spacebars-tests to new-style helpers
Port more helpers to the preferred syntax
Merge branch 'devel' into helper-syntax
Use preferred helper style in docs,apps,packages
History.md for helper syntax
Merge branch 'helper-syntax' into devel

Conflicts:
	History.md