We currently use a mix of C standard includes (e.g. limits.h) and their
C++ variants (e.g. climits). This migrates to a consistent style for all
cases where the C++ variants are acceptable, but leaves the C
equivalents in place where they are required, such as in the embedder
API and other headers that may be used from C.
Make a single-param ctor explicit in order to prevent surprising
implicit conversions.
Add a check for zero message-size and don't malloc/memcpy the incoming
message in those cases.
Add braces where they were missing.
Simplify FlDartProject by removing the path arguments - we'll assume that it's
in the standard bundle layout and add new methods later if we need to support
other cases.
