syntax: replace HirInfo with new Properties type

This commit completely rewrites how HIR properties are computed
inductively.

Firstly, 'Properties' is now boxed, so that it contributes less space to
each HIR value. This does add an allocation for each HIR expression, but
most HIR expressions already require at least one alloc anyway. And
there should be far fewer of them now that we collapse literals
together.

Secondly, 'Properties' now computes far more general attributes instead
of hyper-specific things. For example, instead of 'is_match_empty', we
now have 'minimum_len' and 'maximum_len'. Similarly, instead of
'is_anchored_start' and 'is_anchored_end', we now compute sets of
look-around assertions found anywhere, only as a prefix and only as a
suffix.

We also remove 'is_line_anchored_{start,end}'. There were only used in
the 'grep-regex' crate and they seem unnecessary. They were otherwise
fairly weird properties to compute.

Author: BurntSushi