WIP: Initial progress on the Shackle runtime implementation

[Dekker1]

This PR is very much a work in process, made available here to track the testing of the branch to ensure I keep the level of testing up-to-scratch.

Must haves:

• Parameter value representation
  • Integers (includes Boolean) |boxed + unboxed|
  • Floats |boxed + unboxed|
  • Strings |boxed|
  • Sequences (e.g., tuples and array) |boxed|
  • Int/Float sets |boxed|
  • Sequence Views (e.g., slices, indexed, or transposed sequences) |boxed|
• Decision variable representation
  • Boolean
  • Integer
  • Float
  • Set of Integers
• Initial interpreter functionality
• Propagator design

Maybe part of this, or in future PRs

• Optimize data parsers to directly produce runtime values
• Solver interfacing

[codecov]

Codecov Report

Attention: Patch coverage is 87.11230% with 241 lines in your changes missing coverage. Please review.

Project coverage is 76.12%. Comparing base (aee37cd) to head (4b31838).

Files with missing lines	Patch %	Lines
crates/shackle-runtime/src/value/set.rs	87.39%	70 Missing and 3 partials ⚠️
crates/shackle-runtime/src/value/seq.rs	85.47%	65 Missing and 4 partials ⚠️
crates/shackle-runtime/src/value.rs	88.30%	30 Missing and 1 partial ⚠️
crates/shackle-runtime/src/value/num.rs	96.98%	12 Missing and 3 partials ⚠️
crates/shackle-runtime/src/value/bool_var.rs	0.00%	11 Missing ⚠️
crates/shackle-runtime/src/value/float_var.rs	0.00%	11 Missing ⚠️
crates/shackle-runtime/src/value/int_set_var.rs	0.00%	11 Missing ⚠️
crates/shackle-runtime/src/value/int_var.rs	0.00%	11 Missing ⚠️
crates/shackle-runtime/src/builtin.rs	0.00%	7 Missing ⚠️
crates/shackle-runtime/src/error.rs	0.00%	1 Missing ⚠️
... and 1 more

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #106      +/-   ##
===========================================
+ Coverage    75.64%   76.12%   +0.48%     
===========================================
  Files          124      134      +10     
  Lines        42263    44133    +1870     
  Branches     42263    44133    +1870     
===========================================
+ Hits         31968    33597    +1629     
- Misses        9572     9802     +230     
- Partials       723      734      +11     

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