fix ACSL unicode operators

[kroening]

The ACSL unicode operators are UTF-8, not UTF-16; also add missing ACSL
operators.

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[allredj]

Passed Diffblue compatibility checks (cbmc commit: 00c8512).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/88365396

[martin-cs]

Is ACSL now fully supported / documented / encouraged / working / a work-in-progress / of future interest / mainly of historical interest / deprecated?