Parser: Fix infix or prefix operator rule

InfixOprPrefixOp fragment rule is part of both prefixOp and infixOp
parser rules.  This causes an application expression:

  let x = 1 + 2

to be parsed as a constant expression:

  let x = 1

because the '+' token does not match the infixOp parser rule.

Signed-off-by: Pekka Enberg <penberg@iki.fi>
Parser: Fix application expression rules

Fix application expression parser rule to include the infix operator and
drop a redundant rule with parenthesis.

Signed-off-by: Pekka Enberg <penberg@iki.fi>
AST: Application expression operators

Add plus and minus operators to application expressions.  This
duplicates the functionality of OperatorStringifier which needs to be
addressed add some point.

Signed-off-by: Pekka Enberg <penberg@iki.fi>
Codegen: Application expressions

Add code generation for simple integer application expressions.  You can
now evaluate simple expressions in the interactive shell:

  > let x = 1 + 2
  val x = 3

  > let x = 1 - 3
  val x = -2

Signed-off-by: Pekka Enberg <penberg@iki.fi>
Update Maven POM

Signed-off-by: Pekka Enberg <penberg@iki.fi>