Replaced concat with push for performance

[Balearica]

Description

Replaces instances of d = d.concat(...) with push within encoding functions for improved performance per #512. Should not impact results.

Motivation and Context

This version is faster, particularly on Firefox. As a test, I ran window.font.download() in the console of the https://opentype.js.org/ site. In Firefox the change reduced runtime from 1,341ms to 190ms. Chrome experienced a more modest improvement from 203ms to 166ms. The ability to quickly generate custom font files is important for my project Scribe OCR, and I'm sure others as well.

How Has This Been Tested?

The unit tests pass and I am using this version in my live site. Additionally, the change is conceptually simple and narrow in scope.

Types of changes

• [x ] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I did npm run test and all tests passed green (including code styling checks).
• I have added tests to cover my changes.
• My change requires a change to the documentation.
• I have updated the README accordingly.
• I have read the CONTRIBUTING document.

[yne]

It seems related to the fact that
.concat() create an intermediate array
.push() mutate the existing array

Very nice improvement 👍