Require parenthesis around yield expressions

If yield is used in an expression context parenthesis are now required.
This ensures that the code is unambiguos.

Yield statements c

Require parenthesis around yield expressions

If yield is used in an expression context parenthesis are now required.
This ensures that the code is unambiguos.

Yield statements can still be used without parenthesis (which should be
the most common case).

Also yield expressions without value can be used without parenthesis,
too (this should be the most common case for coroutines).

If the yield expression is used in a context where parenthesis are required
anyway, no additional parenthesis have to be inserted.

Examples:

// Statements don't need parenthesis
yield $foo;
yield $foo => $bar;

// Yield without value doesn't need parenthesis either
$data = yield;

// Parentheses don't have to be duplicated
foo(yield $bar);
if (yield $bar) { ... }

// But we have to use parentheses here
$foo = (yield $bar);

This commit also fixes an issue with by-ref passing of $foo[0] like
variables. They previously weren't properly fetched for write.

Additionally this fixes valgrind warnings which were caused by access to
uninitialized memory in zend_is_function_or_method_call().

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