Make SubExpression

[BFergerson]

Following is valid under current bnf:

define
application sub entity
  key app_uuid blahblah
  has app_name;

Currently parsed as:

define expression
 - expression
  - sub expression
 - expression
  - key expression
 - expression
  - has expression

As opposed to something more intuitive:

define expression
 - sub expression
  - key expression
  - has expression

Will need to determine all the different types of expressions and the valid hierarchies they can have.

[BFergerson]

Use these:

• https://github.com/graknlabs/grakn/tree/master/grakn-graql/src/main/java/ai/grakn/graql/internal/pattern/property
• https://github.com/graknlabs/grakn/tree/f1a69d26ff7f2ce51364e9db3c0b4986bc34ca0a/grakn-core/src/main/java/ai/grakn/graql

[BFergerson]

Even found a grammar file:

• https://github.com/graknlabs/grakn/blob/master/grakn-graql/src/main/antlr4/ai/grakn/graql/internal/antlr/Graql.g4

[BFergerson]

Ended up with a close copy to Graql.g4.