Remove the "cdylib" crate-type from all proc-blocks

[Michael-F-Bryan]

As part of #19 we added crate-type = ["cdylib", "rlib"] to the [lib] section of each proc-block's Cargo.toml file, letting us compile the crate directly to a *.wasm file that can be loaded at runtime.

This works well when you build a proc-block normally (e.g. cd normalize && cargo build) or when generating a *.wasm file for the purpose of extracting metadata (cargo build --target=wasm32-unknown-unknown --features=metadata). However, when the crate is used as a dependency without the metadata feature (i.e. as part of rune build), we try to generate the cdylib and fail because it expects certain language items.

error: no global memory allocator found but one is required; link to std or add `#[global_allocator]` to a static item that implements the GlobalAlloc trait

error: `#[alloc_error_handler]` function required, but not found
note: Use `#![feature(default_alloc_error_handler)]` for a default error handler

error: `#[panic_handler]` function required, but not found

This PR removes the crate-type declarations and modifies cargo xtask dist to build each crate using cargo rustc --crate-type=cdylib. It should be good to merge, I'm just waiting for rust-lang/cargo#10388 (fixed 14 hours ago) to hit nightly so we get the fix for rust-lang/cargo#10356.

This also moves the cargo xtask dist step in CI to its own job because the --crate-type argument requires nightly Rust and the normal tests are compiled/run on the host while cargo xtask dist cross-compiles to WebAssembly.