Represent names using string rather than array of parts

In most circumstances we are interested in the whole string, not
the parts split by namespace separator. As names are common, this

Represent names using string rather than array of parts

In most circumstances we are interested in the whole string, not
the parts split by namespace separator. As names are common, this
representation measurably improves memory usage and performance.

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Produce error if <?= used as identifier

Due to a peculiarity of how <?= is handled, it can get treated as
an echo identifier. Make sure it isn't, to match PHP behavior.