Add type restrictions to methods in Adapter::Base

[SlayerShadow]

Currently there are errors in adapters when compiling against Crystal built from master:

• Error: abstract def Granite::Adapter::Base#clear(table_name) must be implemented by Granite::Adapter::Pg
• Error: abstract def Granite::Adapter::Base#insert(table_name, fields, params, lastval) must be implemented by Granite::Adapter::Pg
• Error: abstract def Granite::Adapter::Base#update(table_name, primary_name, fields, params) must be implemented by Granite::Adapter::Pg
• Error: abstract def Granite::Adapter::Base#delete(table_name, primary_name, value) must be implemented by Granite::Adapter::Pg

For other adapters there are the same errors.

This PR fixes the problem. Because of subclasses already define all these methods with type restrictions, there should not be breaking changes.

Tested against:

Crystal 0.36.0-dev [7e0fdb0] (2021-01-18)

LLVM: 11.0.0
Default target: x86_64-unknown-linux-musl

(crystal-lang/crystal@7e0fdb0a)

[robacarp]

thanks @SlayerShadow

[SlayerShadow]

Never mind - that's already in master.