Drop {assertthat} dependency

[jameslamb]

Description

{assertthat} is a strong dependency of this {uptasticsearch}. It's a collection of small functions that provide some syntactic sugar and nice error messages for common assertions.

{uptasticsearch} should drop that dependency.

Benefits of this work

• reduces the risk of needing to update under thread of CRAN archival if / when {assertthat} is eventually fully abandoned
• reduces the number of packages that need to be installed to use {uptasticsearch}
• removes a GPL-3 dependency from {uptasticsearch}, which could be a problem in some environments that are way of copyleft licenses

Acceptance criteria

• {uptasticsearch} does not depend on {assertthat}

Approach

We have good tests we can trust, try a change and see what happens.

Try to replace these with internal helper functions.

Notes

{assertthat} appears to be abandoned.

• last commit was almost 6 years ago: https://github.com/hadley/assertthat/commits/master/
• last release was almost 6 years ago: https://cran.r-project.org/web/packages/assertthat/index.html
• last time a maintainer responded to an issue was 4+ years ago: https://github.com/hadley/assertthat/issues