Rename steps to functions

Renames step-related types to use function terminology throughout.

Changes

• runner.Step interface → runner.Function
• function.ExecutableStep → function.Exec
• function.SequenceOfSteps → function.Steps
• function.StepFunction → function.Builtin
• function.LazilyLoadedStep → function.Step
• bldr.FixedResultStep → bldr.FixedResultFunction

File Renames

• step.go → function.go
• step_function.go → builtin.go
• executable_step.go → exec.go
• sequence_of_steps.go → steps.go
• lazily_loaded_step.go → step.go
• step_builder.go → function_builder.go