feat/fix/chore: various improvements

[G4Vi]

Resolves #14

feat:

• add Plugin::reset
• implement WasmSourceBytes

fix:

• Manifest: serialize header values correctly
• current_plugin - when outputting, check against output range correctly
• Function namespace constructor - actually construct

chore:

• protect various class members
• move non-initializer list constructors out of header
• reduce copying with std::move
• bump to C++ 17
• remove ManifestKey, the stl containers and std::optional are sufficient
• use std::unique_ptr to more safely manage Plugin's ExtismPlugin lifetime
• make std::string no longer required for plugin calls. The duplication introduced can be removed with std::string_view if sdk: consider switching away from null terminated strings to ptr and length extism#646 is accepted and implemented
• make jsoncpp a private dependency. This improves build times and makes the cpp-sdk not opinionated about what json library the user uses, if any.
• use size_t instead of ExtismSize when referring to items in native memory / not using libextism

[zshipko]

Looks good!