Implement minimal support for generic buffers

[kyouko-taiga]

This PR lets us write this:

public fun main() {
  let x = Array([1, 2, 3])
  print(x[0])
}

The Array constructor used in this program has the following signature:

public init<n: Int>(_ elements: sink Element[n])

n is a generic value parameter that determines the size of the buffer taken as argument. In the example, inference is able to deduce the value of this parameter from the size of the buffer.

[codecov]

Codecov Report

Attention: Patch coverage is 87.47300% with 58 lines in your changes missing coverage. Please review.

Project coverage is 88.24%. Comparing base (4d264ce) to head (22a95a4).
Report is 34 commits behind head on main.

Files with missing lines	Patch %	Lines
...ources/FrontEnd/TypeChecking/SubstitutionMap.swift	81.48%	15 Missing ⚠️
...urces/FrontEnd/TypeChecking/ConstraintSystem.swift	86.66%	14 Missing ⚠️
Sources/FrontEnd/TypeChecking/TypeChecker.swift	91.40%	11 Missing ⚠️
Sources/FrontEnd/TypeChecking/Solution.swift	55.55%	4 Missing ⚠️
Sources/FrontEnd/Terms/TermVariable.swift	50.00%	3 Missing ⚠️
Sources/FrontEnd/Terms/AnyTerm.swift	91.30%	2 Missing ⚠️
Sources/FrontEnd/Terms/ErrorTerm.swift	0.00%	2 Missing ⚠️
Sources/FrontEnd/Terms/TermProtocol.swift	81.81%	2 Missing ⚠️
Sources/FrontEnd/Terms/ConcreteTerm.swift	75.00%	1 Missing ⚠️
Sources/FrontEnd/Terms/GenericTermParameter.swift	80.00%	1 Missing ⚠️
... and 3 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1571      +/-   ##
==========================================
+ Coverage   88.18%   88.24%   +0.06%     
==========================================
  Files         374      380       +6     
  Lines       22709    22801      +92     
==========================================
+ Hits        20025    20121      +96     
+ Misses       2684     2680       -4     

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