Major upgrade! See commit log for details.

List of changes, from most important to less:

- Rename almost all functions: aCall is now call, aReturn is now return, etc. Structures (aStack, aPath, aStep, etc.) still start with a.
- Rewrite the README completely.
- Add at-parameters! Now you don't need to write lambdas in the aStep to access the aStack.last (or other variables you set in the aStack).
- e () (the error reporting function) now joins arguments through space instead of printing the array, so that the error message comes in a si
ngle line. I can't believe I didn't this before.
- You can omit passing an aStack to call, cond, fork and stop, instead of having to pass an undefined aStack to them in the initial asynchronous call.
- Examples are now bundled within aFunctions, and run in sequence.
- Define aPest as either an aStep or aPath
- fork and stop now can take an aPest instead of an aPath.
- You can pass a number as the copy argument to return and it will be stringified automatically.