Implement a "fast path" for rendering: no type checks, maybe no `Node`?

[davepeck]

Currently:

1. Our processor checks the type of every interpolation on every call to html(); if the developer knows that the types of interpolations will never vary for two templates with the same .strings tuple, we should offer a fast path that cuts out the extraneous checks.

2. html() returns a Node tree, even though often all we want is a str

This issue asks us to consider and address both.

/cc @ianjosephwilson

[davepeck]

By the way, I wonder if this stipulation:

types of interpolations will never vary for two templates with the same .strings tuple,

may be too strong? If we want developers to be able to separately control the behavior even for two tuples with the same .strings tuples, we'll need to expose extra machinery to them.

[davepeck]

Also: the question of whether and when to check open/close component interpolation values probably gets sucked up into this too.