nloptr 2.1.0

This release deprecates the default behavior of the inequality equations in any
wrapper function which uses them. Currently, they are calibrated to be >= 0.
This version allows for the equations to be consistent with the main nloptr
function, which requires <= 0. In a future release, the default behavior will
switch to assuming the calibration is <= 0, and eventually, the >= 0 behavior
will be removed. It also includes a large number of safety and efficiency
changes, and an expansion of the unit tests to 100% coverage for all files but
one. The major changes include:

• Reversed the direction of the inequality equations hin and hinjac in the
  wrapper functions which use them, bringing them into compliance with the main
  nloptr call. This addresses
  Issue #148;
• Cleaned the Hock-Schittkowski problem no. 100, Hartmann 6-dimensional, and
  Powell exponential examples. This addresses
  Issue #152 and
  Issue #156;
• Updated roxygen version;
• Updated maintainer email;
• Deal with NA returns from parallel::detectCores() (contributed by @jeroen in
  PR #150);
• Setup rhub v2 checks;
• Update cmake installation instructions on Mac with brew (#146);
• Allow use of equality constraints with COBYLA (#135);
• Replaced the unit testing framework of testthat with tinytest (See
  Issue #136);
• Brought coverage of is.nloptr to 100%. The only file not completely covered
  by unit tests is nloptr.c. The uncovered calls are error messages which get
  trapped by tests in R before the call gets to C;
• Linted package for code correctness and consistency;
• Updated vignette, DESCRIPTION, and NEWS;
• Updated package website to use bootstrap 5;
• Expanded unit tests: coverage now over 97% with no file below 90%;
• Removed forcing C++11;
• Added safety checks to C code;
• Added many safety and efficiency enhancements to R code;
• Most R code style made self-consistent;
• Updated documentation and messages for accuracy and mathematical formatting
• Updated Github actions;
• Some bugfixes (e.g. in isres or the warning in nl.grad).

Please see the commit logs for more detailed descriptions of the changes.