Cleanup path handling with AsRef<Path>

[antiochp]

Cleanup how we pass paths around for the txhashset backend files.

[jaspervdm]

LGTM, 👍 assuming all tests pass

[antiochp]

Build failed on CI with -

llvm v10.0.0 [Approved]
llvm package files install completed. Performing other installation steps.
Downloading llvm 64 bit
  from 'https://github.com/llvm/llvm-project/releases/download/llvmorg-10.0.0/LLVM-10.0.0-win64.exe'
ERROR: The remote file either doesn't exist, is unauthorized, or is forbidden for url 'https://github.com/llvm/llvm-project/releases/download/llvmorg-10.0.0/LLVM-10.0.0-win64.exe'. Exception calling "GetResponse" with "0" argument(s): "The remote server returned an error: (500) Internal Server Error." 
This package is likely not broken for licensed users - see https://chocolatey.org/docs/features-private-cdn.
The install of llvm was NOT successful.
Error while running 'C:\ProgramData\chocolatey\lib\llvm\tools\chocolateyinstall.ps1'.
 See log for details.

Kicking it off again to see if it fails consistently.

[antiochp]

Looks like a temporary glitch, llvm downloaded successfully this time.