1951: Lower the precedence of the `as` operator. r=matklad a=goffrie
Previously, the `as` operator was being parsed like a postfix expression, and
therefore being given the highest possible precedence. That caused it to bind
more tightly than prefix operators, which it should not. Instead, parse it
somewhat like a normal binary expression with some special-casing.
Fixes#1851.
Co-authored-by: Geoffry Song <goffrie@gmail.com>