Add `parse(Type, str)` method for parsing Types from a string.
Parse all types, including DataTypes, UnionAlls and Unions.
- Support function types: `typeof(+)`
- Support constant isbits type constructors in parse type
- Support `Vector{<:Number}`
- Fix `Vector{S} where {Int<:T<:Number, S<:T}`
- Add a bunch more automated test cases for better coverage
- Add tryparse
- Add support for names qualified by non-imported top-level packages.
Mark tests for Union{} and Tuple{} broken
- Add test case for new loaded_modules support