Fix #113: Make generated function names inside arrow-function deterministic in watch-mode runs and in recompiles

[dko-slapdash]

This fixes https://github.com/woutervh-/typescript-is/issues/113

I was debugging the reason of why so many compiled files are changed in our project when a (seemingly unrelated) file change is made, and I found out that typescript-is re-generates the validators code on every change, even when the file isn't changed itself.

Why non-deterministic code generation is a problem? Because the output of tsc may be consumed by other tools (in our case, it's consumed by webpack). So every time something unrelated is changed, webpack has to wake up and do a useless CPU work slowing down everything.

The idea in this PR is to not use type.id directly, but instead have a map of typeId->index inside the context and use that index instead of typeId. Once we see a new typeId, we add a new entry to the map. So in terms of the functionality, this is identical to what we had before, but now the code generated is deterministic.

Before this PR

Notice that the numbers used in generated function names (_194, _11 etc.) are arbitrary and non-deterministic, so when something in the compilation changes during the watch-mode tsc build, these numbers may drift.

After this PR

Notice that numbers are now deterministic and start from 0 (e.g. _0, _3).