Use compile-time assertions to deny ZSTy DSTs

Presently, we deny ZSTy DSTs in our APIs via panicking at runtime. However, the
ZSTiness of a DST is statically detectable and can be denied instead at compile
time. This PR replaces our ZSTy DST panics with compile-time assertions. Doing
gives us the freedom later provide meaningful runtime semantics in such cases.

Partially addresses #325
Closes #1149

Author: jswrenn