move runtime templates into stdlib for bundling with lib files

std/assembly/runtime/README.md

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+AssemblyScript runtimes
+=======================
+
+Default
+-------
+
+```
+$> asc ...
+```
+
+The [default runtime](./default.ts) adds proper support for dynamic memory management and garbage collection to your program.
+
+* [TLSF memory allocator](../allocator/tlsf.ts)
+* [ITCM garbage collector](../collector/itcm.ts)
+
+Arena
+-----
+
+```
+$> asc ... --runtime arena
+```
+
+The [arena runtime](./arena.ts) is just enough to make most language features work, but doesn't have sophisticated support for freeing memory. Useful when prototyping or for simple one-shot modules in that it produces very small modules with minimal overhead.
+
+* [Arena memory allocator](../allocator/arena.ts) with `memory.reset()`
+* No garbage collector
+
+None
+-----------------
+
+```
+$> asc ... --runtime none
+```
+
+[No runtime](./none.ts) features at all. Useful for building low-level modules that do not require language features like managed classes, or if you'd like to compose your own runtime by including a custom memory allocator and garbage collector.
+
+* No memory allocator
+* No garbage collector

std/assembly/runtime/arena.ts

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+import "allocator/arena";
+
+// export { memory };

std/assembly/runtime/default.ts

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+import "allocator/tlsf";
+import "collector/itcm";
+
+// export { memory, gc };