Related: https://github.com/icy-arctic-fox/spectator/issues/46

When defining a matcher outside of the `Spectator` module (custom matcher), `Value(ExpectedType)` can't be resolved.
I suspect this is a Crystal compiler bug, since a derived class should not affect lookups of parent classes like this.
Require statements are added to (hopefully) ensure `Spectator::Value` is defined for the initializer.
Related to https://github.com/icy-arctic-fox/spectator/issues/46

Fixes #78

Fixes #71
Fixes https://github.com/icy-arctic-fox/spectator/issues/45

Using Box.unbox on a nil value with a union type causes:

Invalid memory access (signal 11) at address 0x8

Related Crystal issue: https://github.com/crystal-lang/crystal/issues/11839
Fixes: #76

#51

Any method_missing invocation should return self.

https://github.com/icy-arctic-fox/spectator/issues/44

BUG: trying to upcast Nil (Crystal::NilType) <- NoReturn (Crystal::NoReturnType) (Exception)
  from /crystal/src/compiler/crystal/codegen/cast.cr:668:5 in 'upcast_distinct'
  from /crystal/src/compiler/crystal/codegen/cast.cr:523:15 in 'upcast'
  from /crystal/src/compiler/crystal/codegen/codegen.cr:1369:11 in 'accept'
  from /crystal/src/compiler/crystal/codegen/codegen.cr:2274:9 in 'visit'
  from /crystal/src/compiler/crystal/syntax/visitor.cr:27:12 in 'accept'
...

This is necessary to override the original type instead of creating a new one within the spec.

https://github.com/icy-arctic-fox/spectator/issues/43

Overhaul mock system

See merge request !35