Fix spread operators & redirections

[blindFS]

This PR should fix #98, as well as several easy problems mentioned in #142.

I think I should keep each PR focused for better traceability, so the trickier issues related to identifier and parameters are left for later.

Command

• - val_interpolated as the external command head
  • example: ^$"($cmd)"

Redirection

• - append ops, example 1 o>> foo
• - values redirection, example 1 + 1 o> foo
• - redirect pipes should be single token, example cmd1 e>| cmd2

Parameters/command args

• - spread types

Misc

• - variable as cell_path: $foo.$bar.$baz, it turns out $bar is treated as string instead.

Changes made to the existing test cases

• unquoted node in decl_use renamed to val_string to align with stmt_source
• cases that should fail now simply tagged with :error since we generally don't care where the error occurs

[fdncred]

Nice work, thanks!