refactor substitute!, rename it to partially_inline!, and document it

[jrevels]

This is the kind of function we should eventually have in a "compiler/IR stdlib", but for now we can keep it in Meta.

[jrevels]

Prior to a rebase, one of the CircleCI builds here passed, while the rest of CI failed with various segfaults. Not sure why, but after rebasing, this passes tests fine on Anubis. Hopefully CI agrees...

[jrevels]

FreeBSD tests timed out right after the LibGit2 tests.

Travis 64bit Linux tests failed trying to install dependencies:

⋮
W: Failed to fetch http:https://ppa.launchpad.net/git-core/ppa/ubuntu/dists/trusty/main/i18n/Translation-en  Unable to connect to ppa.launchpad.net:http:
W: Failed to fetch http:https://ppa.launchpad.net/couchdb/stable/ubuntu/dists/trusty/main/binary-amd64/Packages  Unable to connect to ppa.launchpad.net:http:
W: Failed to fetch http:https://ppa.launchpad.net/couchdb/stable/ubuntu/dists/trusty/main/binary-i386/Packages  Unable to connect to ppa.launchpad.net:http:
W: Some index files failed to download. They have been ignored, or old ones used instead.

The command "sudo -E apt-get -yq --no-install-suggests --no-install-recommends --force-yes install ccache libssl1.0.0 bar time g++-5 gfortran-5" failed and exited with 100 during .

CircleCI x86_64 failed with:

Killed
*** This error is usually fixed by running `make clean`. If the error persists, try `make cleanall`. ***
Makefile:202: recipe for target '/home/circleci/project/usr/lib/julia/sys-debug-o.a' failed
make[1]: *** [/home/circleci/project/usr/lib/julia/sys-debug-o.a] Error 1
Makefile:81: recipe for target 'julia-sysimg-debug' failed
make: *** [julia-sysimg-debug] Error 2
make: *** Waiting for unfinished jobs....
Killed
*** This error is usually fixed by running `make clean`. If the error persists, try `make cleanall`. ***
Makefile:201: recipe for target '/home/circleci/project/usr/lib/julia/sys-o.a' failed
make[1]: *** [/home/circleci/project/usr/lib/julia/sys-o.a] Error 1
Makefile:78: recipe for target 'julia-sysimg-release' failed
make: *** [julia-sysimg-release] Error 2
Exited with code 2

All the other builds passed. I don't think these failures are relevant, but I'll request review before merging.