Concretize IndexLens using to_indices (#43)

Should implement #35.  The "problem" with this is that the resulting values are not exactly what you'd write by hand:

```julia
julia> AbstractPPL.concretize(@varname(x.a[1:end, end][:]), x)
x.a[1:2,2][Base.Slice(Base.OneTo(2))]
```

That wouldn't be so much of a problem besides printing, but unfortunately, lenses currenctly compare equality using strict types:

```julia
julia> AbstractPPL.concretize(@varname(x[1:3, :]), rand(10, 10)).lens.indices == @varname(x[1:3, 1:10]).lens.indices
true

julia> AbstractPPL.concretize(@varname(x[1:3, :]), rand(10, 10)).lens == @varname(x[1:3, 1:10]).lens
false

julia> AbstractPPL.concretize(@varname(x[1:3, :]), rand(10, 10)) == @varname(x[1:3, 1:10])
false
```

Cf. jw3126/Setfield.jl#165; the equality comparison can hopefully be fixed there.

The remaining thing is that subsumption must still be able to work with `Colon`, since a user might index a trace/VarInfo using a non-concretized varname containing a colon.  But at least we can then be sure that one side is always concrete.

Co-authored-by: Hong Ge <hg344@cam.ac.uk>
Co-authored-by: Hong Ge <yebai@users.noreply.github.com>

Author: phipsgabler

Project.toml

@@ -3,7 +3,7 @@ uuid = "7a57a42e-76ec-4ea3-a279-07e840d6d9cf"
 keywords = ["probablistic programming"]
 license = "MIT"
 desc = "Common interfaces for probabilistic programming"
-version = "0.5.3"
+version = "0.6"
 
 [deps]
 AbstractMCMC = "80f14c24-f653-4e6a-9b94-39d6b0f70001"
@@ -15,5 +15,5 @@ SparseArrays = "2f01184e-e22b-5df5-ae63-d93ebab69eaf"
 [compat]
 AbstractMCMC = "2, 3, 4"
 DensityInterface = "0.4"
-Setfield = "0.7.1, 0.8"
+Setfield = "0.8.1"
 julia = "~1.6.6, 1.7.3"

bors.toml

@@ -15,4 +15,4 @@ required_approvals = 1
 use_squash_merge = true
 # Uncomment this to use a two hour timeout.
 # The default is one hour.
-timeout_sec = 7200
+timeout_sec = 7200

src/AbstractPPL.jl

@@ -1,7 +1,16 @@
 module AbstractPPL
 
 # VarName
-export VarName, getsym, getlens, inspace, subsumes, varname, vsym, @varname, @vsym
+export VarName,
+    getsym,
+    getlens,
+    inspace,
+    subsumes,
+    subsumedby,
+    varname,
+    vsym,
+    @varname,
+    @vsym
 
 
 # Abstract model functions