Make `Loader` object-safe #196

# to view logs

Job	Run time
no_and_all_features (x86_64-unknown-linux-gnu)	29s
no_and_all_features (wasm32-unknown-unknown)	30s
feature_tests (ubuntu, handlebars, walkdir)	20s
feature_tests (ubuntu, handlebars, ignore)	18s
feature_tests (ubuntu, tera, walkdir)	19s
feature_tests (ubuntu, tera, ignore)	20s
feature_tests (macos, handlebars, walkdir)	19s
feature_tests (macos, handlebars, ignore)	19s
feature_tests (macos, tera, walkdir)	20s
feature_tests (macos, tera, ignore)	20s
feature_tests (windows, handlebars, walkdir)	22s
feature_tests (windows, handlebars, ignore)	22s
feature_tests (windows, tera, walkdir)	23s
feature_tests (windows, tera, ignore)	22s
	5m 3s

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