Switches from "encoding module" to "default module"

[zslayton]

Builds on PR #872.

Switches encoding directive syntax from:

$ion_encoding::(/*...*/)

to:

$ion::(module _ /*...*/)

in anticipation of multi-module support.

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

[codecov]

Codecov Report

Attention: Patch coverage is 73.21429% with 30 lines in your changes missing coverage. Please review.

Project coverage is 77.61%. Comparing base (c103ca3) to head (3a844cd).

Files with missing lines	Patch %	Lines
src/lazy/system_reader.rs	56.89%	11 Missing and 14 partials ⚠️
src/lazy/encoder/writer.rs	20.00%	0 Missing and 4 partials ⚠️
src/lazy/expanded/compiler.rs	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #873      +/-   ##
==========================================
- Coverage   77.62%   77.61%   -0.02%     
==========================================
  Files         136      136              
  Lines       34092    34143      +51     
  Branches    34092    34143      +51     
==========================================
+ Hits        26465    26499      +34     
- Misses       5659     5666       +7     
- Partials     1968     1978      +10     

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[popematt]

Looks good. Only 2 minor comments about the use of hard coded strings.