This supersedes the `capnp encode` and `capnp decode` commands. It didn't make sense to add JSON to those commands since it was unclear if JSON should be thought of as the "encoded" or "decoded" format. `convert` allows mapping anything to anything.

This command is also useful for, say, converting unpacked format to packed format or vice versa, which can now be done without a schema.

Fixes #405

This is necessary to get things working in MSVC. Note I swapped unistd.h
for miniposix.h in the compiler, too, which will be necessary to port the
compiler to MSVC.

This commit also pulls capnp/compiler/capnp.c++'s pipe() implementation
details (i.e., 8k reserved memory, _O_BINARY mode) into kj/miniposix.h.

In Sandstorm, we are encoding powerbox queries in packed base64 strings which may be placed in URL query parameters or the like. The strings are provided in interfaces called browser-side. We anticipate that some developers will prefer to specify a hardcoded string rather than generate it on-the-fly, since Cap'n Proto is not well-supported in browsers today, and anyway the developer may have no other reason to have a Cap'n Proto dependency at all, and powerbox queries are often static.

In this context, speed is irrelevant, while having a compact encoding is desirable. It felt sad to me to leave in the segment table in this context, adding redundant bytes when we want a compact encoding.

Example:

    const data :Data = embed "some-file.dat";

Files are looked up the same way an import would be.

You can use embed when Data or Text is expected. You can also use it when a struct type is expected -- the file will be interpreted as a message using standard binary serialization.

Distinguishing between "local bugs" and "preconditions" was proving difficult in practice, because a precondition failure in one function may very well indicate a bug in a calling function, but the exception may be thrown through that function, thus when caught the classification is nonsensical. The distinction also was not as useful as imagined. So, I eliminated this distinction.

Add asBytes() and asChars() methods to array classes to reinterpret-cast to bytes / chars, since this happens all the time and is otherwise a huge pain.

Use the new methods in a bunch of places.

Protected by an ifdef to avoid tripping up non-autotools builds

For portions currently copyright by Kenton (most of it), transfer copyright to Sandstorm Development Group, Inc. (Kenton's company).

The license change is practically meaningless, as MIT and BSD 2-clause are legally equivalent. However, the BSD 2-clause license is sometimes confused for its ugly siblings, BSD 3-clause and BSD 4-clause. The MIT license is more immediately recognizeable for what it is.

Rémy Blank and Jason Choy (the two non-trivial contributors) are on record as approving this change:

https://groups.google.com/d/msg/capnproto/xXDd2HUOCcc/gbe_COIuXKYJ

Extend totalSizeInWords() to also return a count of capabilities, which helps when a separate capability table needs to be allocated as well.  Use this in the RPC system.

Fix bug handling absolute source paths with no --src-prefix -- just output to the same directory in this case.

Define new KJ constants for min/max integer values and inf/nan, then remove all use of numeric_limits.

Extend 'capnp const' to support evaluating field default values as well as inner members of a value.

Make 'capnp decode' heuristically detect and warn when the wrong format flag (e.g. --packed, --flat) is used.