Make sure to clear tag word when upgrading a struct list.

Edit error message for the case where copyPointer() hits a FAR pointer.

The existing error message suggests this case can only arise if there is a
a bug in the library. However, malformed input can trigger the error too.
In particular, the error gets thrown when a non-double far pointer resolves
to another far pointer.

fix bug in parsing of non-ascii constants

Fix MSVC build errors

Closes #408. Perhaps the same bug as in commit cff9ba05?

Add HTTP client and server implementation.

The old semantics were intended to allow more implementation freedom, but I find in my actual code it's very convenient to rely on the actual implementation details. Other implementations will just have to deal with it.

Let MSVC build without lite mode

Bootstrap regeneration commit to follow.

Although bootstrap regeneration will basically be a no-op (no bootstraps
have deeply nested generics which this commit fixes), one will follow this
commit for completeness.

MSVC cannot deduce sizes of arrays defined inside class templates.

Bootstrap regeneration commit to follow.

MSVC erroneously refuses to compile code which calculates the address of
objects of static storage duration at compile-time, despite this being
legal. Work around this by changing `_capnpPrivate::brand` from constexpr
pointers to static constexpr functions returning pointers. The compiler
is then free to calculate it at compile-time if it can (gcc, clang), but
can also defer it to run-time if it can't (MSVC).

This change is split into three commits:

1. Changes to the code generator (this commit).
2. A bootstrap regeneration, after which the codebase will not compile.
3. Changes to the generated code support header, after which the codebase
will again compile.

MSVC refuses the following code:

struct Foo { Foo(Foo&) {} };
Foo foo;
[foo] {}();

because it only seems to want to consider const copy constructors when
capturing objects by value in lambda capture lists.