Fixed explicit specialization compiler error of NativeInvokationManager::allocate

[arcana261]

When compiling against clang, the compiler would complain of following error:

error: explicit specialization of
      'allocate' in class scope
       static void * allocate<jvalue *>(jvalue * const values) {

However, since we can't specialize template functions of template
classes outside of them without first specializing said template class
first, a workaround solution is to add a stub template struct and
forward allocate method to it.

[arcana261]

My clang version is:

# fake-jni> clang --version
clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

[Matthewacon]

Thank you for your contribution, however, the prototype version of fake-jni is no longer maintained, see the README. As stated by @meme in #104, clang 9+ is required to compile the prototype.