## Changes
Before this change maps were stored as a regular Go map with string
keys. This didn't let us capture metadata (location information) for map
keys.
To address this, this change replaces the use of the regular Go map with
a dedicated type for a dynamic map. This type stores the `dyn.Value` for
both the key and the value. It uses a map to still allow O(1) lookups
and redirects those into a slice.
## Tests
* All existing unit tests pass (some with minor modifications due to
interface change).
* Equality assertions with `assert.Equal` no longer worked because the
new `dyn.Mapping` persists the order in which keys are set and is
therefore susceptible to map ordering issues. To fix this, I added a
`dynassert` package that forwards all assertions to `testify/assert` but
intercepts equality for `dyn.Value` arguments.
## Changes
The nil value is a real valid value that we need to represent. To
accommodate this we introduced `dyn.KindInvalid` as the zero-value for
`dyn.Kind` (see #904), but did not yet update the comments on
`dyn.NilValue` or add tests for `kind.go`.
This also moves `KindNil` to be last in the definition order (least
likely to care about it).
## Tests
Tests pass.
Pieter Noordhuis