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ExtismPointer can currently refers to three different things:

• a pointer in extism memory
• an offset in input memory
• a pointer in extism memory to the start of data in a memory block

By introducing a new type ExtismHandle for the last item, the api can be made easier to use correctly.

• Add new type to header ExtismHandle
• Get all tests and examples running.
• Update docs
• Make extism length fast again.

PR: #18

It's nice having the extism pdk as a single header. However, the non-imported functions have to be declared static to avoid multiple definitions errors if the pdk is used in multiple c files. This could potentially lead to several copies of the pdk being present in the output wasm.

To keep the PDK as a single header library, but avoid building multiple implementations I think we should follow the model used by projects such as miniaudio.

Protect the implementations with a macro and require in one compilation unit:

#define EXTISM_IMPLEMENTATION
#include "extism-pdk.h"

In all other source files using the pdk, just #include "extism-pdk.h" as you did before.

• namespace IMPORT
• extism_store take const void pointer instead
• add extism_alloc_buf
• rework / rethink extism_alloc_string
• Improve ergonomics and safety of passing strings in general

Make it clear when extism_free is needed or not needed.