Reduce object allocations in error generator.
Reduce object allocations in error generator.

`' (' + string + ')'` allocates 4 strings:

* `' ('`
* The string returned by `' (' + string` (let's call it `str2`)
* `')'`
* The string returned by `str2 + ')'`

In contrast, `" (#{times(index)})"` only allocates one string.

Likewise, `[str1, str2].join` allocates an array only to join
it, where as `"#{str1}#{str2}"` avoids the array allocation.

AEgan/rspec-mocks,rwz/rspec-mocks,maclover7/rspec-mocks,zephyr-dev/rspec-mocks,ducthanh/rspec-mocks,rspec/rspec-mocks,maclover7/rspec-mocks,edwardpark/rspec-mocks,rspec/rspec-mocks,telekomatrix/rspec-mocks,h4ck3rm1k3/rspec-mocks,ducthanh/rspec-mocks,rwz/rspec-mocks,edwardpark/rspec-mocks,h4ck3rm1k3/rspec-mocks,zephyr-dev/rspec-mocks,telekomatrix/rspec-mocks,AEgan/rspec-mocks