templater: remove redundant member variables from templater class We no longer need them since the engine is instantiated in __init__().

templater: resurrect cache of engine instance The engine-level cache was effectively disabled at 48289eafb37d "templater: drop extension point of engine classes (API)" by mistake, which made template rendering quite slow. Spotted by Martin von Zweigbergk.

templater: extract template loader to separate class This avoids reference cycle in the subsequent patch: templater -> _proc -> templater.load -> templater The templater class will be a thin wrapper around the loader and the engine.

tests: rename and document test-command-template.t So that we can specify templater tests as test-template-*. (original) $ wc -l test-command-template.t test-template-* 5041 test-command-template.t 23 test-template-filters.t 5064 total (at this patch) $ wc -l test-template-* 1088 test-template-basic.t 1376 test-template-functions.t 1195 test-template-keywords.t 1760 test-template-map.t 5419 total