ir: make HashCode.toHashString public

[ekiwi]

This will allow chiseltest to save the hash code to disk for the purpose of caching simulation binaries.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you state the API impact?
• Did you specify the code generation impact?
• Did you request a desired merge strategy?

Type of Improvement

• new feature/API

API Impact

• HashCode produced by ir.StructuralHash now has a public str method which returns a string representation of the hash

Backend Code Generation Impact

• none

Desired Merge Strategy

• squash

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (1.2.x, 1.3.0, 1.4.0) ?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you mark as Please Merge?