ConvertFrom/To-Hashtable and/or ConvertTo-PSCustomObject cmdlets

Related discussion in #109, specifically this comment:

Finally got around to reviewing this with @PowerShell/powershell-committee.

It sounds like the problem that's being solved here is to convert nested hashtables to PSCustomObjects when the keys are all strings (when they're not strings, conversion will likely result in key collision due to ToString() returning type names).

To that end, we think it's more generically useful to build a ConvertFrom-Hashtable cmdlet that would allow more generic and custom conversion of hashtables to PSCustomObjects and potentially other types using constructors that expect specific hashtable "schemas". Another possible implementation is to do a ConvertTo-PSCustomObject that does more custom handling (and these aren't mutually exclusive). Additionally, a ConvertTo-Hashtable is probably useful.

We don't have this on our radar right now, but if someone wants to give a go at a ConvertFrom-Hashtable RFC and implementation, go for it.

We think the scenario we're trying to address is primarily casting nested hashtables of hashtables into PSCustomObjects (or possibly even other types).

Start with the related discussion in issue #109, especially the linked comment, to understand the proposed conversion scenarios and unresolved scope. Determine whether the work should cover ConvertFrom-Hashtable, ConvertTo-PSCustomObject, ConvertTo-Hashtable, or multiple cmdlets; done would require a focused RFC ready for PowerShell Committee feedback and a vote.