parse.go: Replace struct fields assignments in parser.go by composite literals

[DQNEO]

When I achieved self-hosting, composite literals were not supported yet, so I had no choice but to write this kind of clumsy field-by-field assignments.

var branchStmt = &ast.BranchStmt{}
branchStmt.Tok = token.Token(tok)
return branchStmt

However, we don't need to do this any more. This can be rewritten in a short form:

return &ast.BranchStmt{
    tok:token.Token(tok),
}

[DQNEO]

Done