Don't call modify() on RubyStrings so eagerly when freezing them

[bbrowning]

This fixes #3019, where in certain cases it's possible to end up with
RubyString instances backed by very large unique ByteLists where only
a tiny portion of the bytes in the ByteList are actually needed.

What was happening is modify was being called on RubyString
instances too eagerly, resulting in unnecessary duplication of their
underlying ByteList instances instead of sharing when possible.

The two changes here are in RubyString#newFrozen and
RubyString#resize and I believe are more correct based on the C
implementation of these methods.

For RubyString#newFrozen, which roughly corresponds to
rb_str_new_frozen in
C (https://github.com/ruby/ruby/blob/v2_2_2/string.c#L932), the logic
looks to me to return the shared string and I don't see it calling
str_make_independent, which is roughly the same thing as
RubyString#modify. So, I just removed the call to modify.

For RubyString#resize, which roughly corresponds to to
rb_str_resize in C, shared strings are only made independent if
their length
differ (https://github.com/ruby/ruby/blob/v2_2_2/string.c#L2155). Thus,
instead of unconditionally calling modify here, I moved it into the
conditions that are true when the length differs.