Rewrite `BorshSchema` to use const functions

[Maksym Zavershynskyi]

Currently BorshSchema has static but not constant methods. These methods when compiled to Wasm occupy significant space. Also, they create a significant execution overhead when self-described borsh deserialization/serialization is called using https://docs.rs/borsh/0.6.2/borsh/schema_helpers/index.html

To fix it we need to implement const version of BorshSchema:: schema_container(). Unfortunately, it means two things:

• We can only use types allowed by the const context;
• schema_container() cannot return a type that requires allocation.

We currently intend to serialize BorshSchemaContainer using either borsh or JSON. Therefore we can have two versions of schema_container():

• schema_container_borsh() -> &[u8];
• schema_container_json() -> &str;
  Both return container in already serialized form. Then we can allow reconstructing BorshSchemaContainer from it, if necessary.
  schema_container_json internally would define const variables for each type and recursively concatenate them using std::concat. As the result, each type will have a compile-time computed schema. Similar technique can be used with byte slices and schema_container_borsh.

This will improve performance in the following way:

• If we want to serialize a self-described borsh type using https://docs.rs/borsh/0.6.2/borsh/schema_helpers/index.html the helper will use schema_container_borsh to prepend an already generated sequence of bytes in front the borsh serialized object. Upon deserialization of that object the helper will check that the schema in the self-described type matches the schema of the type is deserializes it into.
• Wasm will have hardcoded schemas instead of the code that generates them.