Adds write_value to ElementWriter

[popematt]

Issue #, if available:

None

Description of changes:

Adds a write_value(&self, value: &Value) function to ElementWriter so that if you have a &Value you don't need to take ownership and convert to Element in order to easily write the value.

E.g.:

fn foo<W: IonWriter>(value: &Value, writer: &mut W) {
    // Instead of this:
    writer.write_element(&value.to_owned().into());

    // Now we can write this:
    writer.write_value(value);
}

While I was at it, I also updated the documentation for write_element and write_elements, which can be used at any level, not just for top-level values.

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

[codecov]

Codecov Report

Patch coverage: 67.56% and project coverage change: -0.04 ⚠️

Comparison is base (247ac34) 83.34% compared to head (e27cad9) 83.30%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #579      +/-   ##
==========================================
- Coverage   83.34%   83.30%   -0.04%     
==========================================
  Files         104      104              
  Lines       19671    19705      +34     
  Branches    19671    19705      +34     
==========================================
+ Hits        16394    16416      +22     
  Misses       1739     1739              
- Partials     1538     1550      +12     

Impacted Files	Coverage Δ
src/element/writer.rs	71.73% <67.56%> (-4.13%)	⬇️

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[zslayton]

Nice, thanks!

[zslayton]

This makes me wonder if we need/want a write_values(iter) sibling method, but I'm happy to punt on that.