Rust canister release profile tuning

[chenyan-dfinity]

With the current rustc setup, the best Rust release profile for canister seems to be:

[profile.release]
lto = true
opt-level = 2

Observations:

• LTO = true reduces both binary size and cycle cost (#63)
• Changing LTO = true to LTO = 'thin' increases both binary size and cycle cost (#64, #65)
• opt-level = 'z' reduces binary size by 20%+ with cycle cost also increased by 20%+ (#61)
• Changing opt-level = 3 to opt-level = 2 reduces binary size by 5%+ with negligible cycle cost change (#62)