Fix build on non-32/64-bit architectures

[Palladinium]

I've been trying to build rand for AVR (which has 8-bit pointers), failing with errors like the following:

error[E0599]: no method named `wmul` found for type `usize` in the current scope
   --> /home/patrick/.cargo/registry/src/gh.neting.cc-1ecc6299db9ec823/rand-0.8.4/src/distributions/uniform.rs:491:42
    |
491 |                         let (hi, lo) = v.wmul(range);
    |                                          ^^^^ method not found in `usize`
...
561 | uniform_int_impl! { isize, usize, usize }
    | ----------------------------------------- in this macro invocation
    |
    = help: items from traits can only be used if the trait is implemented and in scope
note: `WideningMultiply` defines an item `wmul`, perhaps you need to implement it
   --> /home/patrick/.cargo/registry/src/gh.neting.cc-1ecc6299db9ec823/rand-0.8.4/src/distributions/utils.rs:14:1
    |
14  | pub(crate) trait WideningMultiply<RHS = Self> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

This fixes it, and should fix it for any 8-bit and 16-bit architecture.

[vks]

The changes look fine to me, but we should also consider adding tests for platforms with such small pointer widths.

[josephlr]

The changes look fine to me, but we should also consider adding test for platforms with such small pointer widths.

Agreed, we should have tests for a least one target_pointer_width = "16" architecture. Building for AVR is a bit of a pain right now, due to rust-lang/compiler-builtins#400, which is caused by a bug in LLVM. The upstream support for non-32/64-bit targets has never been that great. So if we test this, we definitely want to pin a toolchain.

Running the following commands worked for me to build rand on the builtin avr-unknown-gnu-atmega328 target:

rustup toolchain install nightly-2021-01-07
rustup default nightly-2021-01-07
cargo build -Z build-std=core --target=avr-unknown-gnu-atmega328 --no-default-features --release

@Palladinium can you add test for this to the CI. See existing no-std build tests:

rand/.github/workflows/test.yml

Line 163 in 52407e7

test-no-std:

We may want to add a dedicated `test-avr job, as it will need its own toolchain.

[josephlr]

I've been trying to build rand for AVR (which has 8-bit pointers),

Note: AVR has 16-bit pointers (and 8-bit registers). See the Wikipedia page and the AVR-Rust Guidebook

[josephlr]

Marking "request changes" for adding to the CI and removing the u8 impls.

[josephlr]

LGTM, thanks for fixing the CI stuff!

[josephlr]

I'll let @vks handle merging/squashing, as I don't normally merge PRs for this repo.

[vks]

Thanks everyone!