Bignum Improvements and Housekeeping

CI

• replaced Travis-CI with GH Actions (b3b218e)

Documentation

• added an example for using a Buffer (:tophat: @steven89) (2e84ac1)
• clarified supported type comments for bignums and IVARs (0ac3bca)
• improved changelog formatting (ed5de05)

Build System

• added standard-version (6da10c8)
• improved "ignore" file extension matching in gitignore (c28ba3e)
• removed codecov dependency (bff5cf9)
• upgraded dependencies and rebuilt package-lock.json (656a075)
• upgraded minimum node engine version to v12.22.9 (a188d2d)