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 86.03604% with 248 lines in your changes missing coverage. Please review.

Project coverage is 72.16%. Comparing base (f0604a8) to head (4207266).

Files	Patch %	Lines
crates/shackle-runtime/src/value/set.rs	87.29%	77 Missing ⚠️
crates/shackle-runtime/src/value.rs	83.27%	46 Missing ⚠️
crates/shackle-runtime/src/value/seq.rs	89.37%	41 Missing ⚠️
crates/shackle-runtime/src/value/int_set_var.rs	0.00%	16 Missing ⚠️
crates/shackle-runtime/src/value/bool_var.rs	0.00%	15 Missing ⚠️
crates/shackle-runtime/src/value/float_var.rs	0.00%	15 Missing ⚠️
crates/shackle-runtime/src/value/int_var.rs	0.00%	15 Missing ⚠️
crates/shackle-runtime/src/value/num.rs	97.24%	12 Missing ⚠️
crates/shackle-runtime/src/builtin.rs	0.00%	9 Missing ⚠️
crates/shackle-runtime/src/error.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #106      +/-   ##
===========================================
+ Coverage    71.13%   72.16%   +1.02%     
===========================================
  Files          126      137      +11     
  Lines        24061    25837    +1776     
===========================================
+ Hits         17116    18644    +1528     
- Misses        6945     7193     +248     

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