fix(c-api) Rename lib's name to wasmer.

This patch does several things.

1. For the crate `wasmer-c-api`, the library name is modified from
   `wasmer_c_api` to `wasmer` in `Cargo.toml`. That way, the new
   library files are named `libwasmer.*` rather than
   `libwasmer_c_api.*`. That's the primaly goal of this patch. The
   rest is a consequence of this point. Why do we want that? Because
   the `build.rs` script of the `wasmer-c-api` crate will configure
   the `soname` (on Linux), the `install_name` + `current_version` +
   `compatibility_version` (on macOS), and the `out-implib` +
   `output-def` (on Windows) for a library named `libwasmer`, which is
   the name we provide in the Wasmer package for the Wasmer
   libraries. If we want everything to be testable, we cannot use
   `libwasmer` in `soname` for a file named `libwasmer_c_api` for
   example. If we rename the file when packaging (as it's done prior
   this patch), we would need to re-update all those information in
   the `Makefile`. It implies to duplicate the code in 2 places. So
   let's do things right directly and only once: We want the library
   to be named `libwasmer`, let's do that.

2. For the crate `wasmer-c-api`, since the library name has been
   renamed to `wasmer`, it creates a conflict with the `wasmer` crate
   which is a dependency. Consequently, this patch also updates the
   `Cargo.toml` file to modifiy the dependency name with the special
   `package` attribute (see
   https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#renaming-dependencies-in-cargotoml
   to learn more). So now, the `wasmer` refers to the `wasmer_c_api`
   crate, and `wasmer-api` refers to the `wasmer` crate.

3. The code of the `wasmer-c-api` crate is updated accordingly. The
   `WasmerEnv` derive procedural macro fails because it expects a
   crate named `wasmer` (which is now `wasmer_api`), so we implement
   the `WasmerEnv` trait by hand.

4. The patch updates the `build.rs` script of the `wasmer-c-api`
   crate:

  1. In the `build_cdylib_link_arg` function: The dependency to the
     `cdylib-link-lines` crate has been removed because the output is
     not exactly the one we expect. So we compute all the
     `cargo:rustc-cdylib-link-arg=…` lines by hand. The version number
     no longer appears in the library file name for example.

  2. In the `build_inline_c_env_vars` function: Values passed to
     `LDFLAGS` have been updated to be `libwasmer` rather than
     `libwasmer_c_api`.

  3. A new `shared_object_dir` helper function has been created
     because it's used in `build_inline_c_env_vars` and in
     `build_cdylib_link_arg`.

5. The `Makefile` has been updated:

  1. In `package-capi`, we no longer rename `libwasmer_c_api` to
     `libwasmer` since the name is correctly defined since the
     beginning now.

     Calling `install_name_tool` on macOS is no longer required since
     `install_name` is correctly set by the linker in the `build.rs`
     script of `wasmer-c-api`.

  2. In `package-docs`, some stuffs have been fixed, like the
     `SOURCE_VERSION` variable that didn't exist, so removed, or the
     `mkdir` command that was incorrect etc.

  3. In `build-docs`, the `wasmer-c-api` crate is excluded from the
     list of crates to generate the documentation for. Mostly because
     the `build-docs-capi` recipe exists, and we must use it to
     generate the documentation of `wasmer-c-api` now.

  4. In `build-docs-capi`, we generate the documentation for the
     `wasmer-c-api` crate. But `rustdoc` uses the library name for the
     directory name in the `target/doc/` directory. Since the library
     name is now `wasmer`, it creates a conflict with the `wasmer`
     crate. Consequently, we update the library name by using `sed` on
     the `Cargo.toml` file before running `cargo doc`, to finally
     restore `Cargo.toml` as it was previously.

lib/c-api/src/error.rs

@@ -63,7 +63,7 @@ thread_local! {
 /// # Example
 ///
 /// ```rust,no_run
-/// # use wasmer_c_api::error::{update_last_error, CApiError};
+/// # use wasmer::error::{update_last_error, CApiError};
 ///
 /// update_last_error(CApiError {
 ///     msg: "Hello, World!".to_string(),