use arrow functions where possible

[VIKTORVAV99]

By converting all functions to arrow functions we optimize two things. First we can directly remove some return statements by using implicit return and secondly it allows bundlers to compress mupltiple const declarations into a single const declaration with multiple constants.
Example:

const test1 = "test1"
const test2 = "test2"
const test3 = "test3"

Becomes

const test1 = "test1", test2 = "test2", test3 = "test3"

Which saves some bytes that would otherwise be used for the function keyword in the pervious version.

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• commit message and code follows the Developer's Certification of Origin

[coveralls]

coverage: 97.143% (+0.03%) from 97.11%
when pulling fd41f98 on VIKTORVAV99:use_arrow_functions_where_possible
into 5c3c3eb on i18next:master.