Allow option to opt-out of provided memory allocator

[HCastano]

This takes #1415 one step further and provides the
no-allocator feature to the top level ink crate, allowing contract authors to opt-out
of the provided memory allocator.

It also adds an example showing how a contract author could provide their own allocator.

Closes #1414.

cc @kvinwang

[codecov-commenter]

Codecov Report

Merging #1661 (ebbac25) into master (b8862a1) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1661      +/-   ##
==========================================
+ Coverage   70.72%   70.74%   +0.01%     
==========================================
  Files         206      206              
  Lines        6416     6416              
==========================================
+ Hits         4538     4539       +1     
+ Misses       1878     1877       -1     

Impacted Files	Coverage Δ
crates/allocator/src/bump.rs	87.60% <0.00%> (ø)
crates/ink/ir/src/ir/attrs.rs	82.02% <0.00%> (+0.28%)	⬆️

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[cmichi]

Lemme be the guy commenting about something on the order of 80 CPL 😬 .

[ascjones]

Can we configure rustfmt to catch this? I think it enforces code CPL to 100, not sure about the comments.

[HCastano]

We already enforce this to 90 characters, which is what it's at 😉

https://github.com/paritytech/ink/blob/b8862a1147a8fb577c82304ed1dec9a8c45c386a/.rustfmt.toml#L1

Edit:

Actually, we should be enforcing comments as 80

https://github.com/paritytech/ink/blob/b8862a1147a8fb577c82304ed1dec9a8c45c386a/.rustfmt.toml#L9

So maybe there is something to look into here 🤔

But everything else is under 100 which is reasonable imo.

[cmichi]

LGTM. Just the nitpicks about having less CPL throughout.

[cmichi]

Add the example to https://github.com/paritytech/ink-examples as well, please.