Add astcatalog and use it in gen-ast-pos

[makenowjust]

This PR introduces a new internal utility package named astcatalog.
Package astcatalog provides a catalog of AST node types obtained from ast/ast.go.

This PR rewrites gen-ast-pos to use astcatalog instead of regexp matching, and this PR also fixes some errors in ast/ast.go docs.

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Important changes for contributors:

• I decide to mark ast.go and ast_*.go as conventional. This means that these files are used for automatic generation (e.g., gen-ast-pos), and so we need to take care of the structures of these files.
  • Thus, I renamed const.go to ast_const.go.
• The conventions to be followed are written at the top of these files.