Implement proper exception handling about asterius HOT 7 CLOSED

Finally! We no longer need to allocate a huge heap upon startup; both the nursery and the object pool can grow on demand, and the scheduler gracefully handles HeapOverflow ret code. I'd say we're also not far from a real gc, but leaving that to a future issue.

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Preliminary progress on 1:

• Pulled quite some weeds in the runtime (removing ~500 loc) without breaking a single test.
• We're gradually getting rid of a lot of internal data structure/interfaces of ghc's original runtime (e.g. Task/InCalls, run queues in Capability, etc). We're fine as long as we don't break the absolute essential interfaces used in ghc's emitted cmm code and rts cmm code (e.g. allocate* functions, StgTSO structs, etc)

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As a result of implementing #44, almost all RTS APIs have different type signatures from the original versions:

• Except allocate/createThread, RTS APIs no longer take a Capability as a parameter
• The rts_eval* functions used to return nothing; now they return an i32 thread id
• The rts_getSchedStatus/rts_checkSchedStatus functions now take thread id as a parameter; they no longer inspect a global data structure reused by the scheduler; they only inspect a thread-local storage. This is critical to prevent thread state leakage (a likely cause for scheduler-related bugs in todomvc)

Before next merge to master, we should also update the docs on rts api, along with the second blog post.

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For a more uniform styled RTS API, we should avoid passing Capability in allocate and createThread* as well. However, quite a few rts cmm files call those functions. What should we do?

• Patch rts cmm files to avoid passing extra parameter
• Implement a hack in the codegen so when encountering such an unsafe ccall, simply wipe the first parameter
• Implement a layer of wrapper in Asterius.Builtins so the exported interfaces in js don't pass it, but the internal functions do

I'll try the last approach first

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Simplified runtime & RTS API is merged to master via 497a7e5. Rest of today's work:

• Handle stg_gc*/stg_raisezh runtime primitives, so when a gc occurs or a Haskell exception is thrown, it's possible to get detailed error information, instead of a ThreadKilled status code with no attached context.

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Handling stack overflow requires dealing with chunked stacks and underflow frames, adding extra difficulty for this issue. We'll just allocate a large stack upon createThread, panic on stack overflow (and hope it doesn't). Heap overflow is simpler to deal with.

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It's been a long time. We have gc and exceptions now.

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