Removed changing constant lookup scope in compiled templates.

[printercu]

It singificantly simplifies code and removes extra lines with
wrong source locations from backtrace.

Without this commit backtrace is:

test.str:13:in `block in singleton class'
test.str:7:in `instance_eval'
test.str:7:in `singleton class'
test.str:5:in `__tilt_70221264454100'
.../tilt/lib/tilt/template.rb:165:in `call'
...

After:

test.str:13:in `__tilt_70182127403480'
.../tilt/lib/tilt/template.rb:165:in `call'

It's especially confusing when used with real files, because locations point to other source.
This can also be avoided by using method_source.lines.map(&:strip).join('; ') to concat lines. But I think it's not a big problem to use self.class::CONST instead of just COST even explicit constant lookup.

[judofyr]

This is a breaking change which I'm not quite ready to make. Some people might actually use proper namespacing and depend on constant lookup being relative to self (which is quite sensible).