Add memory operation syscalls (backport #16447)

[mergify]

This is an automatic backport of pull request #16447 done by Mergify.
Cherry-pick of 2b50529 has failed:

On branch mergify/bp/v1.6/pr-16447
Your branch is up to date with 'origin/v1.6'.

You are currently cherry-picking commit 2b5052926.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   programs/bpf/Cargo.toml
	modified:   programs/bpf/build.rs
	modified:   programs/bpf/rust/mem/Cargo.toml
	new file:   programs/bpf/rust/mem/src/entrypoint.rs
	modified:   programs/bpf/rust/mem/src/lib.rs
	new file:   programs/bpf/rust/mem/tests/lib.rs
	new file:   programs/bpf/rust/membuiltins/Cargo.toml
	new file:   programs/bpf/rust/membuiltins/src/lib.rs
	modified:   sdk/program/src/lib.rs
	new file:   sdk/program/src/program_memory.rs
	modified:   sdk/program/src/program_stubs.rs

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   programs/bpf/Cargo.lock
	deleted by us:   programs/bpf/rust/sysvar/tests/lib.rs
	both modified:   programs/bpf/tests/programs.rs
	both modified:   programs/bpf_loader/src/syscalls.rs
	both modified:   sdk/src/feature_set.rs

To fix up this pull request, you can check it out locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

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[codecov]

Codecov Report

Merging #18149 (fef4caa) into v1.6 (e259388) will decrease coverage by 0.0%.
The diff coverage is 21.4%.

@@            Coverage Diff            @@
##             v1.6   #18149     +/-   ##
=========================================
- Coverage    82.6%    82.5%   -0.1%     
=========================================
  Files         423      424      +1     
  Lines      117150   117271    +121     
=========================================
+ Hits        96797    96834     +37     
- Misses      20353    20437     +84