Handle ambiguous parsing of return type / restricted type

[turbolent]

(Reported by @MaxStalker in https://forum.onflow.org/t/breaking-changes-in-cadence-parser/485)

Problem

The following code parses and executes successfully:

fun main():Int { return 2 }

However, removing the whitespace between the return type and the opening curly brace results in a parsing error:

pub fun main():Int{ return 2 }

Parsing failed: unexpected token: got decimal integer, expected ',' or '}'

Acceptance Criteria

The second example should parse like the first.