Resize capacity for fstring

[jhawthorn]

https://bugs.ruby-lang.org/issues/15960

When a string is #frozen, it's capacity is resized to fit (if it is much larger), since we know it will no longer be mutated we'd like to save the space.

> puts ObjectSpace.dump(String.new("a"*30, capacity: 1000))
{"type":"STRING", "class":"0x7feaf00b7bf0", "bytesize":30, "capacity":1000, "value":"...
> puts ObjectSpace.dump(String.new("a"*30, capacity: 1000).freeze)
{"type":"STRING", "class":"0x7feaf00b7bf0", "frozen":true, "bytesize":30, "value":"...

(ObjectSpace.dump doesn't show capacity if capacity is equal to bytesize)

Previously, if we dedup into an fstring, using String#-@, capacity would not be reduced.

> puts ObjectSpace.dump(-String.new("a"*30, capacity: 1000))
{"type":"STRING", "class":"0x7feaf00b7bf0", "frozen":true, "fstring":true, "bytesize":30, "capacity":1000, "value":"...

This commit makes rb_fstring call rb_str_resize, the same as rb_str_freeze does.