Boost libraries - trimmed down for yuzu

This is a subset of Boost v1.71.0 generated using the bcp tool. To get a list of boost modules guaranteed to exist, check the build script.

Updating this repo (on Windows)

To update the Boost version (or to add a new library) follow these steps:

Download Boost and extract the package, then launch Powershell and cd to the boost_1_xx_0 directory.
Build the bcp tool:
```
.\boostrap.bat
.\b2 tools\bcp
```
Store the boost directory in a variable for later use: $boost_dir = $pwd.
Add bcp to your path: $env:Path += $boost_dir\bin.v2\tools\bcp\msvc-14.1\release\link-static\threading-multi (The correct output path should be printed by b2 during the build.)
cd to this repo's directory (...\externals\boost\)
Remove the existing boost from the repo: rm -r boost (This is only necessary if doing a Boost version upgrade, in case they removed any files in the new version.)
Run .\build.cmd $boost_dir to build a new trimmed down distro.
Remove libs\ and Jamroot if you didn't introduce any new boost dependencies that aren't header only
Add/remove all files in git and commit.