Add env variable to Python builds to force debug builds (Qiskit#876)

* Add env variable to Python builds to force debug builds

This commit updates the setup.py for the python package to add a new env
variable `RUST_DEBUG` to force the package to be built in debug mode. By
default pip install will build the rust code in release mode, which is
the sane default you want for publishing or installing a package. But
for local development you typically want to build normally in debug
mode. This increases the build speed and also adds additional runtime
checking to validate the code is fully working. The tradeoff with this
though is the runtime is very poor because the compiler doesn't do any
optimization.

As part of this commit the tox configuration is updated to default to
debug builds. For unit tests the execution time is unchanged, because
while it compiles faster that is offset by the slower execution of the
tests. However, in general I think it's better to run tests in debug
mode by default because it will do runtime validation (e.g. bounds checks
overflow detection, etc) which is good to catch in testing.

* Apply suggestions from code review

Co-authored-by: Ivan Carvalho <8753214+IvanIsCoding@users.noreply.github.com>

---------

Co-authored-by: Ivan Carvalho <8753214+IvanIsCoding@users.noreply.github.com>

setup.py

@@ -12,6 +12,11 @@
 from setuptools_rust import Binding, RustExtension
 
 
+# If RUST_DEBUG is set, force compiling in debug mode. Else, use the default    behavior of whether
+# it's an editable installation.
+rustworkx_debug = True if os.getenv("RUSTWORKX_DEBUG") == "1" else None
+
+
 def readme():
     with open('README.md') as f:
         return f.read()
@@ -25,7 +30,7 @@ def readme():
 PKG_PACKAGES = ["rustworkx", "rustworkx.visualization"]
 PKG_INSTALL_REQUIRES = ['numpy>=1.16.0']
 RUST_EXTENSIONS = [RustExtension("rustworkx.rustworkx", "Cargo.toml",
-                                 binding=Binding.PyO3)]
+                                 binding=Binding.PyO3, debug=rustworkx_debug)]
 
 retworkx_readme_compat = """# retworkx
 

tox.ini

@@ -10,6 +10,7 @@ setenv =
   LANGUAGE=en_US
   LC_ALL=en_US.utf-8
   ARGS="-V"
+  RUST_DEBUG=1
 deps =
   setuptools-rust
   fixtures
@@ -23,6 +24,7 @@ extras =
 passenv =
   RETWORKX_TEST_PRESERVE_IMAGES
   RUSTWORKX_PKG_NAME
+  RUSTWORKX_DEBUG
 changedir = {toxinidir}/tests
 commands =
   stestr run {posargs}
@@ -44,12 +46,14 @@ commands =
 basepython = python3
 setenv =
   {[testenv]setenv}
+  RUSTWORKX_DEBUG=1
 deps =
   -r {toxinidir}/docs/source/requirements.txt
 passenv =
   {[testenv]passenv}
   RETWORKX_DEV_DOCS
   RETWORKX_LEGACY_DOCS
+  RUST_DEBUG
 changedir = {toxinidir}/docs
 commands =
   python -m ipykernel install --user