Alias type

[VojtechVitek]

Introduced in Peter's "v2" branch

• https://github.com/webrpc/webrpc/tree/schema-v2
• v0.8 with schema updates #134
• schema v2, aka webrpc-gen v0.8.0 #123

alias Balance: string
  + go.field.type = BigInt
  + go.tag.db = balance
  + json = b_a_l_a_n_c_e
  
struct User
  - id: uint64
  - username: string
  - amount: Balance

[pkieltyka]

Some work has been done in #227 -- however, please see PR comments which requires more work on an approach which won't require any changes to generators (which is what we'd want)