support for view and module loading by start and run commands (#31)

This PR implements support for loading views and making modules available inside an environment via command line arguments to `start` and `run` commands.

```bash
# start with the view named develop loaded
uenv start --view=develop myenv

# start with modules available
uenv start --modules myenv

# run a command with modules available
uenv run --modules myenv -- bash -c "module load cray-mpich; mpicc test.c"

# run a command with a view loaded
uenv run --view=develop myenv -- mpicc test.c
```

* add `lib/envvars.py` that  implements classes that support setting and modifying environment variables.
    * support for `scalar` variables that take a single value and `list` variables (e.g. `PATH`) that are assembled by setting, appending and prepending paths.
    * support for accumulating operations on collections environment variables
    * support 
* instead of sourcing `activate.sh` when loading a view, the script is parsed and `export` commands is generated based its contents
    * this allows us to perform operations like overriding or preserving existing list variables
    * in the future this can be used to generate information required to deactivate a view
* changed the behavior of `uenv view` to preserve existing values of variables like `PATH`, `LD_LIBRARY_PATH`, etc
* add unit tests for the new environment variable code

VERSION

@@ -1 +1 @@
-4.1.1
+5.0.0-dev

activate

@@ -5,6 +5,7 @@ export UENV_CMD='env -u PYTHONPATH -u VIRTUAL_ENV @@impl@@'
 export UENV_IMG_CMD='env -u PYTHONPATH -u VIRTUAL_ENV @@image_impl@@'
 export UENV_VERSION=@@version@@
 export UENV_PREFIX=@@prefix@@
+export UENV_WRAPPER_CMD=@@wrapper@@
 
 function uenv {
     local _last_exitcode=$?

install

@@ -157,6 +157,7 @@ echo "local install: $local_install"
 libexec=${prefix}/libexec
 impl_path="${libexec}/uenv-impl"
 img_impl_path="${libexec}/uenv-image"
+wrapper_path="${libexec}/uenv-wrapper"
 lib_path="${libexec}/lib"
 if [ "$local_install" == "no" ]
 then
@@ -169,6 +170,7 @@ echo "installing $lib_path"
 run install -v -m755 -d "$lib_path"
 run install -v -m644 -D -t "${destdir}/${lib_path}" ${script_path}/lib/alps.py
 run install -v -m644 -D -t "${destdir}/${lib_path}" ${script_path}/lib/datastore.py
+run install -v -m644 -D -t "${destdir}/${lib_path}" ${script_path}/lib/envvars.py
 run install -v -m644 -D -t "${destdir}/${lib_path}" ${script_path}/lib/jfrog.py
 run install -v -m644 -D -t "${destdir}/${lib_path}" ${script_path}/lib/names.py
 run install -v -m644 -D -t "${destdir}/${lib_path}" ${script_path}/lib/oras.py
@@ -179,6 +181,9 @@ run install -v -m644 -D -t "${destdir}/${lib_path}" ${script_path}/lib/terminal.
 echo "installing ${img_impl_path}"
 run install -v -m755 -D "$script_path/uenv-image" "${destdir}/${img_impl_path}"
 
+echo "installing ${wrapper_path}"
+run install -v -m755 -D "$script_path/uenv-wrapper" "${destdir}/${wrapper_path}"
+
 echo "installing $impl_path"
 run install -D -m755 <(sed "s|@@version@@|$version|g" $script_path/uenv-impl) ${destdir}/$impl_path
 
@@ -206,6 +211,7 @@ run install -v -m644 -D <(
         -e "s|@@version@@|$version|g" \
         -e "s|@@prefix@@|$prefix|g"   \
         -e "s|@@image_impl@@|$img_impl_path|g" \
+        -e "s|@@wrapper@@|$wrapper_path|g" \
         $script_path/activate) \
       $init_path