Try out TypeScript

[searls]

Progress

• install typescript
• rip out babel
• move our existing typings test in with the other tests, deleting its npm script & the generated directory
• see npm test pass
• verify the dist/testdouble.js looks okay
• see the whole build pass
• set up a second standard task for linting typescript
• convert an actual JS file to typescript
• measure build performance

Blockers

• Currently using ts-node for its require.extensions hook to run our tests, but it appears to blow up on Node v4 which is failing the build. (Fix: the explosion was caused by me targeting es2015 instead of es5)

[searls]

I'm deciding to merge for two reasons:

1. It seems to mostly work
2. It's much faster than under babel.

In particular, $ time npm t takes this long under Node v8.90 on master:

real	0m9.226s
user	0m7.091s
sys	0m0.998s

And here's the same command compiled under typescript:

real	0m1.945s
user	0m1.566s
sys	0m0.189s

For a better than 4x speedup in walltime for my test feedback loop, I'd gladly switch compilers even if I never write a single '.ts' file.