Upgrade default rust-toolchain to 1.56-beta.3

[sffc]

This unblocks various PRs that need to use .map_project() and related functions (such as #1084). It also unblocks us to start updating the Yoke and DataPayload APIs to turn on examples and remove obsolete functions like with_context (#1061).

We will switch to 1.56 stable on October 21, and we will not push anything to crates.io unless the code builds on stable.

[sffc]

Hmm

---- src/trait_hack.rs - trait_hack (line 31) stdout ----
Test compiled successfully, but it's marked `compile_fail`.

failures:
    src/trait_hack.rs - trait_hack (line 31)

test result: FAILED. 25 passed; 1 failed; 2 ignored; 0 measured; 0 filtered out; finished in 3.15s

I'll fix it.

[sffc]

OK, CI is happy. Need another approval.

[Manishearth]

Can we also unmark the other yoke examples as compile_fail? And perhaps file a followup to get rid of the trait hack

[sffc]

I'll follow up with another PR shortly to address #1061 and get the Yoke/DataPayload examples in order.

[sffc]

(merging based on Zibi's previous approval)

[Manishearth]

wfm