BUG/PERF: Fixed IntervalIndex.nbytes and itemsize
Avoid materializing the ndarray of Intervals. Just get from left and right
instead.

Closes https://github.com/pandas-dev/pandas/issues/19209
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20600?src=pr&el=h1) Report
> Merging [#20600](https://codecov.io/gh/pandas-dev/pandas/pull/20600?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/aa3fefc898b2f101f3cf83f90add69857926fcbe?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20600/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20600?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20600      +/-   ##
==========================================
+ Coverage   91.84%   91.84%   +<.01%     
==========================================
  Files         153      153              
  Lines       49256    49260       +4     
==========================================
+ Hits        45241    45245       +4     
  Misses       4015     4015
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <100%> (ø)` | :arrow_up: |
| #single | `41.91% <50%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20600?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/20600/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `93.13% <100%> (+0.04%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20600?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20600?src=pr&el=footer). Last update [aa3fefc...8bff87c](https://codecov.io/gh/pandas-dev/pandas/pull/20600?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Sorry, was just using this to demo stuff for a talk on OSS I gave last night.

I think this will be solved by Interval being an ExtensionArray (should have that PR today). Closing in favor of #20611.