Explore enabling `large-tables` and/or `huge-tables` by default

[sgrif]

I was looking at the compile times for our test suite with various table size features enabled, both when Diesel needed to be compiled, and when only the tests needed to be compiled. This was done on rustc 1.22.0-nightly (325ba23d5 2017-09-19), with incremental comp enabled.

# Baseline
> (cd diesel_tests && touch tests/lib.rs && cargo build --tests --features postgres)
   Compiling diesel v0.16.0 (file:///Users/sean/Documents/Projects/open-source/diesel/diesel)
   Compiling diesel_infer_schema v0.16.0 (file:///Users/sean/Documents/Projects/open-source/diesel/diesel_infer_schema)
   Compiling diesel_tests v0.1.0 (file:///Users/sean/Documents/Projects/open-source/diesel/diesel_tests)
   Compiling diesel_codegen v0.16.0 (file:///Users/sean/Documents/Projects/open-source/diesel/diesel_codegen)
    Finished dev [unoptimized + debuginfo] target(s) in 71.95 secs
>  (cd diesel_tests && touch tests/lib.rs && cargo build --tests --features postgres)
   Compiling diesel_tests v0.1.0 (file:///Users/sean/Documents/Projects/open-source/diesel/diesel_tests)
    Finished dev [unoptimized + debuginfo] target(s) in 24.19 secs

# `large-tables` turned on                                                                                                                                                                                       > (cd diesel_tests && touch tests/lib.rs && cargo build --tests --features postgres)
   Compiling diesel v0.16.0 (file:///Users/sean/Documents/Projects/open-source/diesel/diesel)
   Compiling diesel_infer_schema v0.16.0 (file:///Users/sean/Documents/Projects/open-source/diesel/diesel_infer_schema)
   Compiling diesel_tests v0.1.0 (file:///Users/sean/Documents/Projects/open-source/diesel/diesel_tests)
   Compiling diesel_codegen v0.16.0 (file:///Users/sean/Documents/Projects/open-source/diesel/diesel_codegen)
    Finished dev [unoptimized + debuginfo] target(s) in 77.74 secs
> (cd diesel_tests && touch tests/lib.rs && cargo build --tests --features postgres)
   Compiling diesel_tests v0.1.0 (file:///Users/sean/Documents/Projects/open-source/diesel/diesel_tests)
    Finished dev [unoptimized + debuginfo] target(s) in 25.0 secs

# `huge-tables` turned on
> (cd diesel_tests && touch tests/lib.rs && cargo build --tests --features postgres)
   Compiling diesel v0.16.0 (file:///Users/sean/Documents/Projects/open-source/diesel/diesel)
   Compiling diesel_infer_schema v0.16.0 (file:///Users/sean/Documents/Projects/open-source/diesel/diesel_infer_schema)
   Compiling diesel_tests v0.1.0 (file:///Users/sean/Documents/Projects/open-source/diesel/diesel_tests)
   Compiling diesel_codegen v0.16.0 (file:///Users/sean/Documents/Projects/open-source/diesel/diesel_codegen)
    Finished dev [unoptimized + debuginfo] target(s) in 147.86 secs
> (cd diesel_tests && touch tests/lib.rs && cargo build --tests --features postgres)
   Compiling diesel_tests v0.1.0 (file:///Users/sean/Documents/Projects/open-source/diesel/diesel_tests)
    Finished dev [unoptimized + debuginfo] target(s) in 24.12 secs

It looks like these features no longer have any effect on application compilation time, and the large-tables feature might have a small enough effect on the crate's compile time to be worth turning on by default. huge-tables still looks like it doubles our compilation time, but given that most users don't recompile Diesel often, maybe it's worth just enabling that by default, too. I want to explore this more on stable and without incremental comp.

[sgrif]

#1520