Undefine VERSION in gemspec to avoid namespace pollution between mult…

…iple MemoWises

This commit updates `memo_wise.gemspec` to undefine the `MemoWise::VERSION`
constant after it is read, as first suggested in
simplecov-ruby/simplecov#557 (comment).

Before this change, accessing `MemoWise::VERSION` immediately after the
`doff_and_don` call in `benchmarks.rb` gives the version of the gem
pulled from GitHub, even though `doff_and_don` should mean nothing is in
the `MemoWise` namespace. After this change, accessing `MemoWise::VERSION`
there gives this error as we expect:

```
uninitialized constant MemoWise (NameError)
```

memo_wise.gemspec

@@ -1,10 +1,19 @@
 # frozen_string_literal: true
 
+# After loading the VERSION constant, save it to a variable to use in this
+# gemspec and then undefine it. This allows benchmarks to compare multiple
+# versions of MemoWise against each other without inadvertently sharing the same
+# VERSION constant.
+# NOTE: Other uses of this trick often switch `require_relative` to `load`, as
+# in https://github.com/simplecov-ruby/simplecov/issues/557#issuecomment-825171399,
+# but for our purposes `require_relative` seems to work fine. If any issues
+# arise, feel free to use `load` instead.
 require_relative "lib/memo_wise/version"
+gem_version = MemoWise.send(:remove_const, :VERSION)
 
 Gem::Specification.new do |spec|
   spec.name     = "memo_wise"
-  spec.version  = MemoWise::VERSION
+  spec.version  = gem_version
   spec.summary  = "The wise choice for Ruby memoization"
   spec.homepage = "https://github.com/panorama-ed/memo_wise"
   spec.license  = "MIT"