feat: print AST statistics with -Zast-stats flag

[Chriscbr]

Closes #25

For example, running cargo run -- -Z ast-stats examples/src/Counter.sol from the project root emits:

ast-stats AST STATS
ast-stats Name                Accumulated Size         Count     Item Size
ast-stats ----------------------------------------------------------------
ast-stats Block                     32 ( 2.7%)             2            16
ast-stats Type                      64 ( 5.4%)             2            32
ast-stats - Elementary                64 ( 5.4%)             2
ast-stats Stmt                     160 (13.6%)             2            80
ast-stats - Expr                     160 (13.6%)             2
ast-stats Expr                     240 (20.4%)             5            48
ast-stats - Assign                    48 ( 4.1%)             1
ast-stats - Unary                     48 ( 4.1%)             1
ast-stats - Ident                    144 (12.2%)             3
ast-stats Item                     680 (57.8%)             5           136
ast-stats - Variable                 136 (11.6%)             1
ast-stats - Contract                 136 (11.6%)             1
ast-stats - Pragma                   136 (11.6%)             1
ast-stats - Function                 272 (23.1%)             2
ast-stats ----------------------------------------------------------------
ast-stats Total                  1_176
ast-stats

[Chriscbr]

This is just a first pass at adding a form of AST stats. The approach is heavily based on that of rustc; looking for any feedback!

[DaniPopes]

LGTM, thanks!