refactor API to be more consistent with other ulid implementations

[rmalayter]

Right now the dbo.ulid() function returns a UNIQUEIDENTIFIER type, while in ULID implementations for most other languages ulid() returns the string version of a ULID. Refactor API of this implementation to be more consistent with others. It might be best to not have a "bare" dbo.ulid() function at all, and instead always include a "type" as part of the function name for clarity, e.g. dbo.ulidMSSQL() would return the MSSQL-sort-friendly specific format UNIQUEIDENTIFIER, and dbo.ulidChar() would return a CHAR(26) BASE32-encoded version. Add conversion functions as well.