Document performance-sensitive choices in compilation

[workingjubilee]

This includes, and includes rationales for

• Why use cargo check first and then cargo rustc later? Answer: maybe don't?
• Why set CARGO_TARGET_DIR?
• Why use the LTO + 1 CGU profile (as follows) for the build?

[profile.release]
debug-assertions = true
codegen-units = 1
lto = "fat"
opt-level = 3
panic = "unwind"