Consumers would have greater access to mental healthcare and substance abuse treatment under a proposed Massachusetts law. The proposal would lift limits on hospitalizations and outpatient treatment for dozens of maladies, such as eating disorders, post-traumatic stress, and drug addictions. Supporters say the measure would ease the stigma on mental illness and allow patients to receive treatment. But a coalition of business and insurance groups say the bill would disproportionately hurt small employers at a time when they are struggling with rising healthcare costs.