Pipeline cleanup

[KaanOzkan]

When we reach feature parity we need to cleanup the code with the idioms that come up in Stripe C++ reviews:

• Use designated initializers { .foo = foo}

• Prioritize spans instead of creating vectors where possible

• Don't pass vectors into functions. Try to pass pointers or spans

• Reminder: There are some comments in the main draft PR [WIP] Migrate RBS rewriter to handle Prism AST #681

• MethodTypeToParserNodePrism

• TypeToParserNodePrism

• RBSMethodCommon

• CommentsAssociatorPrism

• SigsRewriterPrism

• AssertionsRewriterPrism

• TypeParamsToParserNodesPrism

• SignatureTranslatorPrism

• pipeline_test_runner.cc