codegen: Simpler code for timeseries

[lutter]

When an entity type is declared with @entity(timeseries: true) codegen should produce slightly different code for it:

1. The constructor for such types should take no arguments and automatically set id to "auto" and timestamp to 0 (graph-node will fill in the correct value automatically)
2. All numeric fields should be initialized to 0 (of the appropriate type)

There's also a small feature request for graph-ts in here: it would be good to have constants for commonly used BigDecimal and BigInt values, especially ZERO_BD and ZERO_BI for 0 in those types (or BigDecimal::ZERO, not sure if that can be done in AssemblyScript)