feat: support ToMap for arrays too

[yangtau]

This PR:

• ToMap() is currently only for slices. This PR adds support to arrays.
• Panic when pivot is not a field of the struct.
  Before this PR, the panic message was ambiguous:

  panic: interface conversion: reflect.Type is nil, not *reflect.rtype [recovered]
      panic: interface conversion: reflect.Type is nil, not *reflect.rtype
  

  Now:

  panic: `Id` must be a field of the struct Foo [recovered]
          panic: `Id` must be a field of the struct Foo
  

[thoas]

thank you @yangtau, great changes!