Hello Guix hackers,

Below is a preliminary version of a new guix command called
'environment'.  For those familiar with Nix, it works a lot like
nix-shell.

The purpose of 'guix environment' is to assist hackers in creating
reproducible development environments without polluting their package
profile.  'guix environment' takes a package (or packages), builds all
of the necessary inputs, and creates a shell environment to use them.
For example, 'guix environment guile' adds these environment variables:

  export PATH="/gnu/store/hzq21w0m7akpc3b9gxq9p0vlr74k8pib-pkg-config-0.27.1/bin:/gnu/store/3yiqz9wmwx6b7hpbapg5q39sjx33kh0j-bash-4.3.27/bin:/gnu/store/26vfhzfgl2iqfwa92ij8zqssgc4amh9y-gcc-4.8.3/bin:/gnu/store/2x8wq5w4cxgmz0f982gz37wzdglj0865-ld-wrapper-boot3-0/bin:/gnu/store/4cbvfyx31xyh7pmmvv3w5h0689pcnlnw-glibc-2.20/bin:/gnu/store/4cbvfyx31xyh7pmmvv3w5h0689pcnlnw-glibc-2.20/sbin:/gnu/store/dlg2zv7w3d3bfj5qg7yhskvgj65080p2-binutils-cross-boot0-2.24/bin:/gnu/store/pcizz33yzwyiirfsinbg08v3aha6krx4-make-boot0-4.0/bin:/gnu/store/qgx70g83pab3b08ff6cl0rrzv33bf074-diffutils-3.3/bin:/gnu/store/37w1i5q2iqm0alymc1iiibs12nv87wap-findutils-4.4.2/bin:/gnu/store/1qza3p5n6w5ciphli8wm05jam72jczfd-file-5.19/bin:/gnu/store/jfavabhnk1am05pyivk7ryixznqa49l4-binutils-bootstrap-0/bin:/gnu/store/mbacj3k992756m
 xskp26pd8b476mynhk-bootstrap-binaries-0/bin:/gnu/store/52vg2sr63d5vpz4np3sa1vf5y5gyhnrf-readline-6.3/bin:/gnu/store/z8ifyrrgawhmd5w3v0fdjdv52cwhb0pd-ncurses-5.9/bin"
  export PKG_CONFIG_PATH="/gnu/store/8f9db6s6j77w130y9b13wsnsbalhfrf9-libffi-3.1/lib/pkgconfig:/gnu/store/8g6zcbj2fzzl6xis3mcsmpxrqsfsaxs4-libgc-7.4.2/lib/pkgconfig"
  export CPATH="/gnu/store/26vfhzfgl2iqfwa92ij8zqssgc4amh9y-gcc-4.8.3/include:/gnu/store/4cbvfyx31xyh7pmmvv3w5h0689pcnlnw-glibc-2.20/include:/gnu/store/rw5s9immq72plzajqm8g8i4sfg8z0kyc-linux-libre-headers-3.3.8/include:/gnu/store/1qza3p5n6w5ciphli8wm05jam72jczfd-file-5.19/include:/gnu/store/8f9db6s6j77w130y9b13wsnsbalhfrf9-libffi-3.1/include:/gnu/store/52vg2sr63d5vpz4np3sa1vf5y5gyhnrf-readline-6.3/include:/gnu/store/z8ifyrrgawhmd5w3v0fdjdv52cwhb0pd-ncurses-5.9/include:/gnu/store/3j7h3gz5qh1v0j1xjjb2xxr2b3kqcsv6-libunistring-0.9.4/include:/gnu/store/wz94w6nr5wsvy06inwfpc0pxqp1cpkly-libtool-2.4.2/include:/gnu/store/8g6zcbj2fzzl6xis3mcsmpxrqsfsaxs4-libgc-7.4.2/include:/gnu/store/l61k4nkiw8wc064ynf17l40sfgczyh56-gmp-6.0.0a/include"
  export LIBRARY_PATH="/gnu/store/4cbvfyx31xyh7pmmvv3w5h0689pcnlnw-glibc-2.20/lib:/gnu/store/1qza3p5n6w5ciphli8wm05jam72jczfd-file-5.19/lib:/gnu/store/8f9db6s6j77w130y9b13wsnsbalhfrf9-libffi-3.1/lib:/gnu/store/52vg2sr63d5vpz4np3sa1vf5y5gyhnrf-readline-6.3/lib:/gnu/store/z8ifyrrgawhmd5w3v0fdjdv52cwhb0pd-ncurses-5.9/lib:/gnu/store/3j7h3gz5qh1v0j1xjjb2xxr2b3kqcsv6-libunistring-0.9.4/lib:/gnu/store/wz94w6nr5wsvy06inwfpc0pxqp1cpkly-libtool-2.4.2/lib:/gnu/store/8g6zcbj2fzzl6xis3mcsmpxrqsfsaxs4-libgc-7.4.2/lib:/gnu/store/l61k4nkiw8wc064ynf17l40sfgczyh56-gmp-6.0.0a/lib"

With this environment, you can grab the guile source code and build it,
knowing that all the dependencies have been satisfied.  Furthermore,
this system makes it easy to work on projects that require different
versions of the same software without clashes.  Bye-bye RVM and
virtualenv!

By default, running 'guix environment' spawns a new $SHELL process,
because it is usually what one would want to do.  However, the '--exec'
flag can be used to specify the command to run.

Additionally, the default behavior is to prepend search paths to the
existing environment variable values so that one has access to other
installed software e.g. git.  To clear the existing environment firstf,
the '--pure' flag can be used.

Finally, the '--load' flag can be used to read a package from a file
instead of searching $GUIX_PACKAGE_PATH.

There is no documentation yet.  I wanted to get feedback first before
writing it in case substantial changes are made.  :)

WDYT?