This API is a wrapper around the `wasmer_types::Features` API. A new
function `wasm_config_set_features` allows to define features for the
engine and the compiler.
The new rule is the following:
* `wasm_` for the standard C API,
* `wasmer_` or `wasi_` for the Wasmer non-standard C API.
For all symbols inside the `unstable` module, the renaming `wasm_` to
`wasmer_` is done without deprecations. It was clear that those API
were unstable.
For all the other symbols, symbols have been renamed to `wasmer_` but
the old symbols have been kept with deprecation warnings.
Special note: The `wasm_named_extern_t` type (and associated
functions) was in `wasi` by mistake. Its place was in the `unstable`
module. This patch also fixes that.
The `wasm_declare_vec_*` macros have been updated to support a default
prefix, or a user-defined prefix. It's now possible to write
`wasm_declare_boxed_vec!(foo);` to get all the API prefixed by `wasm_`
(as previously), or `wasm_declare_boxed_vec!(foo, wasmer);` to prefix
with `wasmer_`.
A user not using symbols from the `unstable` module will continue to
get working code, modulo some deprecations, after this patch.
