Add support for generating structs in Rust with code generation

[desaikd]

Add support to generate struct based on Ion Schema. This will include supporting conversion of various struct related constraints in ISL.

Following ISL constraints will be supported with struct generation:

• element
• fields
• type

Action Items for this implementation:

• Define an initial workflow for code generation
• Define templates for struct in Rust
• Define constraint mapping for element, fields and type constraints in ISL to struct in Rust
• Implement code generation for read and write API for struct in Rust

[desaikd]

Mappings for ISL constraints to Rust struct:

element -> SequenceStruct(creates a struct with single field that stores the sequence as a Vec)
e.g.

struct Foo { value: Vec<String> }

fields -> Struct (creates a struct with given field name and value/data type pairs)
e.g.

struct Foo { name: String, id: i64 }

type -> ValueStruct (creates a struct with single field that has the data type provided with type constraint)
e.g.

struct Foo { value: String }

[desaikd]

Completed with #61 and #81