Improve use of unsafe Rust in arena.rs (#2391)

[mthom]

The use of unsafe Rust is revised in arena.rs to answer some of the criticisms made in #2391, particularly regarding the misaligned allocation of some stream types. Explicit use of alloc is removed in favor of Box. TypedAllocSlab<Payload> is a type parameterized template over AllocSlab used to allow rustc to get value allocation right. As well, some new macros (like mem::offset_of!) are used to calculate byte offsets into structs. This requires bumping the Rust version in Cargo.toml to 1.77.

[triska]

Several new style check warnings:

warning: this `if` statement can be collapsed
   --> src/machine/system_calls.rs:431:17
    |
431 | /                 if heap[h].get_tag() == HeapCellValueTag::PStrOffset {
432 | |                     if heap[h_offset].get_tag() == HeapCellValueTag::CStr {
433 | |                         return if pstr_chars < max_steps {
434 | |                             CycleSearchResult::ProperList(pstr_chars + 1)
...   |
439 | |                     }
440 | |                 }
    | |_________________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_if
    = note: `#[warn(clippy::collapsible_if)]` on by default
help: collapse nested if block
    |
431 ~                 if heap[h].get_tag() == HeapCellValueTag::PStrOffset && heap[h_offset].get_tag() == HeapCellValueTag::CStr {
432 +                     return if pstr_chars < max_steps {
433 +                         CycleSearchResult::ProperList(pstr_chars + 1)
434 +                     } else {
435 +                         let offset = max_steps as usize + n;
436 +                         CycleSearchResult::PStrLocation(max_steps, h_offset, offset)
437 +                     }
438 +                 }
    |

error: read amount is not handled
   --> src/parser/char_reader.rs:332:13
    |
332 |             (&self.buf[self.pos..]).read_vectored(bufs)?
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_io_amount
    = note: `#[deny(clippy::unused_io_amount)]` on by default

warning: match expression looks like `matches!` macro
   --> src/machine/machine_indices.rs:301:56
    |
301 |                       meta_specs.iter().find(|meta_spec| match meta_spec {
    |  ________________________________________________________^
302 | |                         MetaSpec::Colon | MetaSpec::RequiresExpansionWithArgument(_) => true,
303 | |                         _ => false,
304 | |                     })
    | |_____________________^ help: try: `matches!(meta_spec, MetaSpec::Colon | MetaSpec::RequiresExpansionWithArgument(_))`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#match_like_matches_macro
    = note: `#[warn(clippy::match_like_matches_macro)]` on by default

warning: casting to the same type is unnecessary (`usize` -> `usize`)
   --> src/machine/system_calls.rs:436:42
    |
436 | ...                   let offset = max_steps as usize + n;
    |                                    ^^^^^^^^^^^^^^^^^^ help: try: `max_steps`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
    = note: `#[warn(clippy::unnecessary_cast)]` on by default

[triska]

The test build for ubuntu-22.04 with Rust 1.70 now fails, I think this target should now be removed since Rust 1.77 is required?

Run cargo build --all-targets --target x86_64-unknown-linux-gnu  --verbose
error: package `scryer-prolog v0.9.4 (/home/runner/work/scryer-prolog/scryer-prolog)` cannot be built because it requires rustc 1.77 or newer, while the currently active rustc version is 1.70.0

[Skgland]

I think this job should have been changed to use 1.77 instead of being removed. This job is here to test that the version declared in the Cargo.toml still works, which I think is what to # rust version comment directly above it tried to convey.