Windows is much pickier about dllimport/dllexport. Declare it on
the declaration, not the definition. Also ensure that the declaration
precedes the definition. Finally, remove a stray OPENSSL_EXPORT.

Change-Id: Id50b9de5acbe5adf1b15b22dd60b7a5c13a80cce
Reviewed-on: https://boringssl-review.googlesource.com/1862
Reviewed-by: Adam Langley <agl@google.com>

The |size| method was documented to return the same as |ECDSA_size| -
the max size of an ECDSA signature. However, this involves some ASN.1
calculations which is best done once. What custom implementations want
to give is the size of the group order on which the ASN.1 computations
are based.

This change switches the |size| method to allow that.

Change-Id: I95b6e0c2b52bfcd0d74850c2c4e9bc01269255e2
Reviewed-on: https://boringssl-review.googlesource.com/1200
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)