Don't always omit parentheses for argument-less yield

We may need parentheses around an argument-less yield to distinguish
(yield) - $a from yield -$a and similar.

Treat argumen

Don't always omit parentheses for argument-less yield

We may need parentheses around an argument-less yield to distinguish
(yield) - $a from yield -$a and similar.

Treat argument-less yield like a prefix operator. This will print
parentheses a bit more often than is really required, because the
arity ambiguity only exists for certain operators. This seems like
a reasonably good approximation through, as it only affects
argument-less yield on the LHS of infix operators.

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Handle another yield edge case

match also uses =>, so we need to make sure that a trailing yield
is wrapped in parentheses.

Support yield precedence

Since PHP 7.0 yield is a proper expression, so print it with
proper precedence. If the pretty printer is configured for
older version (non-default), then alw

Support yield precedence

Since PHP 7.0 yield is a proper expression, so print it with
proper precedence. If the pretty printer is configured for
older version (non-default), then always print parentheses.

There is an interesting interaction here, in that => is resolved
in favor of yield if the yield occurs as part of an array (or
outer yield). This kind of bypasses the entire precedence hierarchy
and *only* affects yield expressions. For the sake of simplicity
this is modeled via normal LHS precedence, because this will only
add unnecessary parentheses to a handful of low precedence unary
operators. If desired, a special marker for this purpose could be
added though.

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Properly support prefix operator precedence printing

The pretty printer is supposed to produce a minimal-parentheses
printing for expressions. However, for prefix operators, we were

Properly support prefix operator precedence printing

The pretty printer is supposed to produce a minimal-parentheses
printing for expressions. However, for prefix operators, we were
failing to do so in ways that are practically meaningful, e.g.
$a = yield from $b produced redundant parentheses around the
yield from.

For prefix operators, the precedence of the direct parent operator
is not actually relevant: What matters is the precedence of any
infix operator whose LHS it appears on. For example, $a . include $b
does not require parentheses, but (include $a) . $b does.

To handle this, keep separate track of the parent operator
precedence, and a special LHS precedence which is used for unary
operator printing only. Because the LHS precedence may not come
from the direct parent, we have to pass it down the call stack.
And if we do that, we may as well do the same for the parent
precedence.

This also fixes an integration test failure with php-src, because
arrow function precedence was previously not respected (and would
be ugly to respect without this change).

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Remove PHP 5 parser

Introduce Nop statement to collect dangling comments

A Nop statement will be inserted into statement lists if there are
any trailing comments in the list (which would otherwise not be

Introduce Nop statement to collect dangling comments

A Nop statement will be inserted into statement lists if there are
any trailing comments in the list (which would otherwise not be
associated with any node).

The pretty printer output currently still contains a superfluous
newline.

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Pretty printer test coverage

Our output for yield / yield from is currently not very nice, but
also not easy to change.