uc-crux-llvm: User-provided specifications for functions

[langston-barrett]

Fixes #982. Based on #1001. Best place to start reviewing would probably be by reading the user-facing docs (rendered).

[langston-barrett]

Rebased onto #1001.

[langston-barrett]

CI failure seems unrelated:

Run cd dependencies/mir-json && cargo install --locked --force
  cd dependencies/mir-json && cargo install --locked --force
  shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}
  env:
    CACHE_VERSION: 4
    NIX_PATH: nixpkgs=channel:[2](https://github.com/GaloisInc/crucible/runs/7522835904?check_suite_focus=true#step:10:2)1.11
    NS: nix shell github:nixos/nixpkgs/nixos-22.0[5](https://github.com/GaloisInc/crucible/runs/7522835904?check_suite_focus=true#step:10:5)#cabal-install github:nixos/nixpkgs/nixos-22.05#haskell.compiler.ghc902 nixpkgs#gmp nixpkgs#zlib nixpkgs#zlib.dev
warning: Using `cargo install` to install the binaries for the package in current working directory is deprecated, use `cargo install --path .` instead. Use `cargo build` if you want to simply build the package.
  Installing mir-json v0.1.0 (/home/runner/work/crucible/crucible/dependencies/mir-json)
    Updating crates.io index
error: failed to get `cargo` as a dependency of package `mir-json v0.1.0 (/home/runner/work/crucible/crucible/dependencies/mir-json)`

Caused by:
  failed to fetch `https://github.com/rust-lang/crates.io-index`

Caused by:
  error reading from the zlib stream; class=Zlib (5)
Error: Process completed with exit code [10](https://github.com/GaloisInc/crucible/runs/7522835904?check_suite_focus=true#step:10:11)1.

[langston-barrett]

CI seems to be failing reliably at this step, though why that should be so for the Ubuntu 20.04/GHC 9.0.2 build and not the other Ubuntu 20.04 builds is a bit mystifying considering that this step doesn't involve GHC/Cabal.

[robdockins]

I don't really have context for the UC-Crux changes, but the change to crucible proper seem fine to me.