eval builtin can take a block

[andychu]

Part of #631

Just realized that the new typed args means that the val builtin can take a (parsed) block directly, in addition to the old string interface.

So instead of eval $mystr, you can be passed a block as a param, and do eval (myblock)

It also means that the literal eval { echo hi } is equivalent to { echo hi } (shell brace group), which makes sense!

This is turning out nicely!

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I think we still need functions eval_word() (returns string) and eval_expr() (returns value_t) though