Remove extraneous clones in TypeNamed and ViolationChild

[popematt]

Issue #, if available:

None.

Description of changes:

I changed the implementation of ViolationChild so that it's not unnecessarily creating clones of IonValues. Also changed the implementation of TypeNamed so that it doesn't create a new struct and clone the entire type definition every time it validates a value.

My informal performance testing shows a significant improvement. I created a representative schema:

type::{ name: A, type: int }
type::{ name: B,  type: string }
type::{ name: C, type: $any }
type::{
  name: abc,
  fields: {
    a: A,
    b: B,
    c: C,
  }
}

And tested it using a set of values that were all valid for the type abc and another set that were all invalid. These results were measured by running warmups until the time was stable, using an "autorange" approach to determine the number of invocations in each sample, and then collecting and averaging the ops/s for 30 samples.

Dataset	Before (ops/s)	After (ops/s)	Change
valid	1375113	2097905	52%
invalid	865061	1065709	23%

Related PRs in ion-schema, ion-schema-tests, ion-schema-schemas:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov[bot]]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (e80edaa) 83.15% compared to head (f942da5) 83.13%.

Files	Patch %	Lines
...src/main/kotlin/com/amazon/ionschema/Violations.kt	44.44%	4 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #296 +/- ## ========================================== - Coverage 83.15% 83.13% -0.02% ========================================== Files 159 159 Lines 3770 3766 -4 Branches 908 905 -3 ========================================== - Hits 3135 3131 -4 - Misses 357 360 +3 + Partials 278 275 -3 ```

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