add support for arbitrary iterables in #each templates

[antoinep92]

Resolves #339
Moved from meteor/meteor#11477 since observe_sequence was moved.

As discussed in the issue, this extends the range of supported {{#each}} arguments to any object implementing the iterable protocol except strings. To minimize changes and risks, these arguments are converted to Arrays, so the iteration mechanics remains unchanged.
IE is not supported.

[antoinep92]

Note: I managed to pass all tests locally by running meteor update which bumped versions in test-app/.meteor/ (mongo version was an issue) but didn't commit those changes as this will probably have other consequences and is outside the scope of the PR.

[StorytellerCZ]

@antoinep92 Thanks for the heads up. I have that fixed in my PR #330