Support using both nullability and type overrides

[raftario]

Implements support for using both nullability and type overrides at the same time, following the syntax proposed by @mehcode in #489.

• as "ident" emits either DbType or Option<DbType> depending on inferred nullability
• as "ident!" emits DbType regardless of inferred nullability
• as "ident?" emits Option<DbType> regardless of inferred nullability
• as "ident: Type" emits either Type or Option<Type> depending on inferred nullability
• as "ident!: Type" emits Type regardless of inferred nullability
• as "ident?: Type" emits Option<Type> regardless of inferred nullability

At the moment, the wildcard syntax stays the same, and both as "ident!: _" and as "ident?: _" result in a compile time parsing error.

• as "ident: _" emits either try_get or try_get::<Option<_>, _> depending on inferred nullability
• as "ident!: _" emits try_get regardless of inferred nullability
• as "ident?: _" emits try_get::<Option<_>, _> regardless of inferred nullability

Addresses #489

[raftario]

Added support for as "ident!: _" and as "ident?: _" as discussed on Discord.

[mehcode]

Skimming through, this looks awesome. I'm not confident in this part of the codebase though so I'll let @abonander merge this.

After #535 this needs a rebase.