Execution context stack space

[chfast]

This implements segmented stack space inside ExecutionContext. I will add more documentation how it works later. Replaces #529 and #572.

Generally, having unlimited stack space is PITA, but doable.

Before this lands I propose to generalize ExecutionContext:

• move it to execution_context.hpp,
• rename Guard to something like LocalContext,
• rename increment_call_depth() to something like create_local_context().

[codecov[bot]]

Codecov Report

Merging #777 (5c43bf4) into master (3afc039) will decrease coverage by 0.06%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #777      +/-   ##
==========================================
- Coverage   99.24%   99.18%   -0.07%     
==========================================
  Files          79       77       -2     
  Lines       11962    10948    -1014     
==========================================
- Hits        11872    10859    -1013     
+ Misses         90       89       -1     

Flag	Coverage Δ
rust	?
spectests	90.61% <100.00%> (+0.06%)	:arrow_up:
unittests	99.18% <100.00%> (-0.02%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
test/unittests/cxx20_span_test.cpp	100.00% <ø> (ø)
lib/fizzy/execute.cpp	99.53% <100.00%> (+<0.01%)	:arrow_up:
lib/fizzy/execution_context.hpp	100.00% <100.00%> (ø)
lib/fizzy/stack.hpp	100.00% <100.00%> (ø)
bindings/rust/src/lib.rs

[chfast]

Unit tests will be updated later.

[axic]

Replaces #529 and #572.

Can we close those two?