Make value intrinsic smarter

[MaxDesiatov]

Note: This PR depends on and includes #88.

Recent addition of value intrinsic allows XMLCoder to handle last few known edge cases, but still a lot of times the shape of an XML isn't guaranteed to be always the same. E.g. some of the attributes might be present or absent, but an element without attributes is never decoded as a KeyedBox even with a value intrinsic. And vice versa, a KeyedBox with attributes can't be decoded with a primitive type that maps to the element value.

This was previously reported by @Inukinator as an issue in InukVT/Read#2.

Consider this sample XML:

<container>
<foo id="123">456</foo>
<foo id="789">123</foo>
</container>

As of 847d754 this can't be decoded as

struct Container: Codable {
  let foo: [Int]
}

And this xml:

<container>
<foo>456</foo>
<foo>123</foo>
<foo>789</foo>
</container>

can't be decoded with

struct Container: Codable {
  struct Foo: Codable {
    let value: Int
  }

  let foo: [Foo]
}

This is fixed now with added optional down casts that peek into the box structure and map it correctly in these remaining edge cases.

[codecov]

Codecov Report

Merging #89 into master will decrease coverage by 0.37%.
The diff coverage is 53.22%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
- Coverage   77.36%   76.98%   -0.38%     
==========================================
  Files          38       38              
  Lines        2076     2112      +36     
==========================================
+ Hits         1606     1626      +20     
- Misses        470      486      +16

Impacted Files	Coverage Δ
...es/XMLCoder/Decoder/XMLDecoderImplementation.swift	61.59% <53.22%> (-0.91%)	⬇️

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