align grammar with parser by introducing PipeElement

[WindSoilder]

This PR aligns the parser implementation with contributing/grammar.md by
introducing PipeElement and making pipelines/redirections more explicit in
the AST.

This branch is stacked on top of #81.

Changes

• introduce PipeElement { expr, redirection }
• distinguish single expressions from multi-element pipelines
• make pipeline require 2+ elements
• add pipeline_or_expression / pipeline_or_expression_or_assignment
• add explicit pipe nodes:
  • Pipe
  • ErrPipe
  • OutErrPipe
• add explicit redirection nodes:
  • OutRedirect
  • ErrRedirect
  • OutErrRedirect
• store pipeline connectors separately in Pipeline { nodes, nexts }
• extend Call with:
  • has_caret
  • as_alias
• reuse call(true) for alias parsing
• refactor block parsing by splitting out statement_sequence

Grammar alignment

Notable grammar updates:

• statement now routes through pipeline_or_expression_or_assignment
• pipeline now means 2+ pipe_elements
• let / mut accept pipeline_or_expression
• postfix now works on simple_expression
• simple_expression replaces the older broader primary role in this area