gohcl: WithRange[T] for concise decoding with source locations

[apparentlymart]

Until now gohcl has forced callers to decide between two non-ideal options:

• Decode directly into a "normal" Go type, like string, but then have no access to the source location of that value.
• Decode into an hcl.Expression and then separately decode that expression into a normal type, which gives access to the source location but requires an extra decoding step downstream.

Source location information is important if subsequent code does any further validation of the value beyond what HCL can do itself, and so this forces implementers to decide between returning low-quality error messages or writing more lines of code for decoding, and since a lot of people would prefer to write less code despite the consequences, this has therefore led to software with poor-quality error messages.

Go 1.18 provides a compromise: we can use a generic struct type to pair up any normal value with an hcl.Range value, thus still achieving a one-shot decode behavior for simpler applications but giving those applications convenient access to both the resulting value and the source location it came from.

This is essentially just another "special" type that gohcl has bespoke handling for, similar to hcl.Body, hcl.Expression, and hcl.Attributes. If decoding an expression into a value of a WithRange[T] type then it behaves as if decoding into a T directly, but writes the result into the Value field of the WithRange[T] alongside the source range.

An important goal here is to do this without requiring all callers of this module to use Go 1.18. To achieve that, the generic code is all in a conditionally-compiled file, and we assume that any caller which includes WithRange[T] would inherently require 1.18 itself anyway, so we won't enter the codepath for handling this type on older versions of Go.

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This is just a prototype for now. It needs some more thorough testing with various interesting variations to see if it's behavior is reasonable in each case. For example:

• WithRange[cty.Value]
• *WithRange[Foo] vs. WithRange[*Foo] for optional attributes.
• WithRange[hcl.Expression] (redundant, but should either work or return an explicit error explaining why not)
• Is it reasonable to use WithRange[T] for a field representing a nested block type, rather than one representing an attribute? Should the range in that case be the block's DeclRange, or something else?
• Is it reasonable to use WithRange[T] on a ,remain-tagged field? (Probably not, but it should still return a sensible error message if someone tries.)
• ...?

[hashicorp-cla]

All committers have signed the CLA.

[apparentlymart]

I'm no longer working at HashiCorp after today, so I won't be able to interact with the branch of this PR anymore and so I'm going to close it but hope it'll still be a useful starting point for a future PR with a similar goal!