"<<" cannot be used in string literals

When << is used inside a string literal, Ion throws a ion: unterminated quote in script error.

Reproduction:

echo "a>>b"  # Ok
echo "a<<b"  # Fails

Build information:

rustc -V: rustc 1.33.0-nightly (4c2be9c97 2019-01-22)

git rev-parse HEAD: 918e1fc4