feat/fix/chore: various improvements (#16)

* fix: header cleanup and fixes to manifest and current plugin

manifest - serialize the header value correctly
current_plugin - when outputting, check against output range correctly

* feat: Plugin::reset, std::move with plugin

* fix: Function namespace constructor - actually construct

* protect Buffer class members

* chore: remove ManifestKey, the size of the stl containers and std::optional is sufficient to check if an item is empty

* chore: use std::unique_ptr to manage plugin lifetime instead of destructor. prevents copying Plugin

* chore: make std::string no longer required for plugin calls

* feat: implement WasmSourceBytes

* chore: make jsoncpp a private dependency

* chore: move Plugin::CancelHandle::cancel out of header

* chore: only use ExtismSize with libextism api / use size_t when referring to items in native memory

* fix: callback on osx?

* chore: return false in CurrentPlugin::Output if out of bounds, std::move and std::string_view cleanup