[ENHANCEMENT+CLEANUP] Better compile time errors for strict mode

[chancancode]

Removing support for implicit this fallback {{foo}} -> {{this.foo}}

When a strict mode template references an undefined variable, it really should produce a compile-time error, arguably that is the point of the feature.

However, it seems like previously a lot of these errors ended up propagating and is actually handled by the runtime compiler.

The original/primary goal here started out as cleaning up all the code for implicit this fallback, but as I delete code it eventually revaled all the places where strict mode was inappropiately using the same infrastructure ("unresolved free variables", which isn't really a sensible thing).

Also:

• Removes {{#with}}
• Removes "deprecated call" @foo={{bar}}
• Removes unused opcodes
• Syncs keyword list with Ember