[JITLink] Add convenience methods for creating block readers / writers.

This saves clients some boilerplate compared to setting up the readers and
writers manually.

To obtain a BinaryStreamWriter / BinaryStreamReader for a given block, B,
clients can now write:

auto Reader = G.getBlockContentReader(B);

and

auto Writer = G.getBlockContentWriter(B);

The latter will trigger a copy to mutable memory allocated on the graph's
allocator if the block is currently marked as backed by read-only memory.

This commit also introduces a new createMutableContentBlock overload that
creates a block with a given size and zero-filled content (by default --
passing false for the ZeroInitialize bypasses initialization entirely).
This overload is intended to be used with getBlockContentWriter above when
creating new content for the graph.

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