Improve rdataset/rrset repr to include rdata.

Improve rdataset/rrset repr to include rdata.

Previously, repr() of rdataset/rrset looked like this:
<DNS IN A rdataset>
<DNS IN TXT rdataset>
<DNS IN RRSIG rdataset>

<DNS example. IN A RRset>
<DNS example. IN TXT RRset>
<DNS example. IN RRSIG(NSEC) RRset>

With this patch, it they look like:
<DNS IN A rdataset: [<1.2.3.4>, <5.6.7.8>]>
<DNS IN TXT rdataset: [<"foo" "bar">, <"baz">]>
<DNS IN RRSIG(NSEC) rdataset: [<NSEC 1 3 3600 20200101000000 20030101000000 2143 foo MxFcby9k/yvedMfQgKzhH5er0Mu/vILz 45IkskceFGgiWC...>]>

<DNS example. IN A RRset: [<1.2.3.4>, <5.6.7.8>]>
<DNS example. IN TXT RRset: [<"foo" "bar">, <"baz">]>
<DNS example. IN RRSIG(NSEC) RRset: [<NSEC 1 3 3600 20200101000000 20030101000000 2143 foo MxFcby9k/yvedMfQgKzhH5er0Mu/vILz 45IkskceFGgiWC...>]>

Note that each rdata is truncated to 100 characters.