refactor: make compile/validate state explicit, off context

[lestrrat]

• Replace the context.Context value-bag with explicit compileState/evalState carriers and option/v3 options on Compile/Validate
• schema.Resolver methods take explicit base scope; all public schema.With*/*FromContext helpers removed (breaking)
• Compiled validators are now immutable config, safe to call concurrently (-race tested); ctx honored for cancellation in compile and validate
• Builder API and generated meta/meta_gen.go unchanged; full 2020-12 suite, -race, and golangci-lint all green
• ctx now carries only cancellation and the trace logger