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 +1,25 @@
 # frozen_string_literal: true
 
-require_relative "lib/memo_wise/version"
+# 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: It's important that we use `load` instead of `require_relative` because
+# the latter only loads a file once, and so by undefining the `VERSION` constant
+# we can make it difficult to access that value again later. For more context,
+# see: https://github.com/panorama-ed/memo_wise/pull/370#issuecomment-2560268423
+# NOTE: If we ever bump the minimum Ruby version to 3.1+, we can simplify this
+# code and use this instead:
+#
+#   spec.version = Module.new.tap do |mod|
+#     load("lib/memo_wise/version.rb", mod)
+#   end::MemoWise::VERSION
+load "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"