Fix alignment

[Wonshtrum]

Fdb Arenas allocate structs not aligned, even though fdb_c.h explicitly specifies a 4bytes alignment for most of them (the others using their default alignment, 8bytes in general):

foundationdb-rs/foundationdb-sys/include/710/fdb_c.h

Lines 88 to 92 in e846355

	#pragma pack(push, 4)
	typedef struct key {
	const uint8_t* key;
	int key_length;
	} FDBKey;

These alignment constraints are translated in Rust in bindings.rs by bindgen and exported by foundationdb_sys:

foundationdb-rs/foundationdb-sys/build.rs

Lines 114 to 115 in e846355

	bindings
	.write_to_file(out_path.join("bindings.rs"))

#[repr(C, packed(4))]
#[derive(Debug, Copy, Clone)]
pub struct key {
    pub key: *const u8,
    pub key_length: ::std::os::raw::c_int,
}
pub type FDBKey = key;

And inherited by their "local wrappers":

foundationdb-rs/foundationdb/src/fdb_keys.rs

Lines 156 to 158 in e846355

	#[repr(transparent)]
	/// An FdbKey, owned by a FoundationDB Future
	pub struct FdbKey(fdb_sys::FDBKey);

Data is transferred between the fdbserver and foundationdb-rs via pointers which lead to this kind of code:

foundationdb-rs/foundationdb/src/fdb_keys.rs

Lines 42 to 49 in e846355

	fn deref(&self) -> &Self::Target {
	assert_eq_size!(FdbKey, fdb_sys::FDBKey);
	assert_eq_align!(FdbKey, fdb_sys::FDBKey);
	unsafe {
	&*(std::slice::from_raw_parts(self.keys, self.len as usize)
	as *const [fdb_sys::FDBKey] as *const [FdbKey])
	}
	}

self.keys is a C pointer that points to a FDBKey potentially misaligned in a Fdb Arena. In the newer versions of Rust from_raw_parts panics with debug_assertions on pointer misalignment. Even if we ignore this, the fact that misaligned structs are currently read correctly is probably due to x86 compliance but is still undefined behavior.

In this PR, "local wrappers" are annotated with packed instead of transparent:
https://github.com/Wonshtrum/foundationdb-rs/blob/c6b198ddef85ca77bab40ea11cda1c08c9d09e35/foundationdb/src/fdb_keys.rs#L157-L159

This explicitly tells Rust that we do not expect these structs to be aligned. This removes the panics on newer versions as these structs are 1byte align (which a pointer will always be) and furthermore, we do not have to rely on x86 to magically accept unaligned structs as misalignment will be explicitly taken care of at machine code generation.

These wrappers are used everywhere in favor of their raw counterparts from fdb_sys, and deref functions are rewritten using this form:
https://github.com/Wonshtrum/foundationdb-rs/blob/c6b198ddef85ca77bab40ea11cda1c08c9d09e35/foundationdb/src/fdb_keys.rs#L46-L50

note also the assert_eq_align!(FdbKey, u8);

[codecov-commenter]

Codecov Report

Patch coverage: 94.73% and project coverage change: -0.17 ⚠️

Comparison is base (e846355) 79.84% compared to head (c6b198d) 79.68%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
- Coverage   79.84%   79.68%   -0.17%     
==========================================
  Files          27       27              
  Lines        5181     5207      +26     
==========================================
+ Hits         4137     4149      +12     
- Misses       1044     1058      +14     

Impacted Files	Coverage Δ
foundationdb/src/mapped_key_values.rs	45.04% <88.88%> (+1.00%)	⬆️
foundationdb/src/fdb_keys.rs	60.93% <100.00%> (+1.92%)	⬆️
foundationdb/src/future.rs	80.43% <100.00%> (+0.10%)	⬆️

... and 12 files with indirect coverage changes

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