Support prefixed types in ClassInjector.SystemTypeFromIl2CppType

[ds5678]

Resolves #135

[js6pak]

ClassInjector.RewriteType already tries to tackle the Il2Cpp prefix but only does so for the System.* types. It also only works for types that have a non-prefixed version which no longer has to be the case since #39.
Thinking about it more I think RewriteType is inherently flawed and it should actually just operate on the Il2CppTypeStruct not System.Types.

[ITR13]

Can confirm that this fixes an bug similar to #135 in another game/mod. The bug occurred for me on MelonLoader 0.7.1, which uses Il2CppInterop 1.5.0-ci.625.

[ds5678]

As a temporary fix until v2