when lookup fail in the current context, go to parent contexts

[gasche]

(This PR is on top of #48 to avoid conflicts in the rendering implementation.)

There was a bug in a corner case of the variable-lookup semantics,
which is important because it corresponds to a fairly natural "if
variable exists" pattern (in non-strict-mode). Consider the following
example when var is mapped to a scalar value in the context:

{{#var}} show the value {{var}} of the defined variable var {{/var}}

The current ocaml-mustache implementation will fail: when entiring the
var section, it would define the lookup context as the scalar value
itself, and any name lookup within the section would fail.

Instead the Mustache documentation and specification require that if
lookup in the current context fails (here the current context defines
no name), then "parent contexts" should be looked up; in particular,
the context before the section was entered, in which the lookup
succeeds.

A {{name}} tag in a basic template will try to find the name key in
the current context. If there is no name key, the parent contexts
will be checked recursively. If the top context is reached and the
name key is still not found, nothing will be rendered.

The present PR implements this "recursive lookup" semantics. Note that
falling back to parent contexts is not allowed in the middle of
resolving a dotted name (we find the right context for the first part
of the name, and then all subsequent lookups happen in
this context). This is explicitated in the Mustache specification,
interpolation.yml, example "Dotted Names - Broken Chain Resolution".

(It is unfortunate that the current specification does not cover the
edge case of scalar sections; I proposed a PR to fix this in
mustache/spec#114 .)

Note: an equivalent template would already work

{{#var}} show the value {{.}} of the defined variable var {{/var}}

but it arguably results in less readable templates for non-experts.

[rgrinberg]

Looks good. A CHANGES entry would be helpful.

@gasche I added you as a collaborator. Feel free to merge approved PR's.

[gasche]

Thanks! I rebased with a change entry, will merge if the CI passes.