Draft: Improve DekuUpdate

[wcampbell0x2a]

• Allow DekuUpdate::update to have ctx
• Add call_update, to call update() on self.
• Add update_custom to call custom function

This is more a bike-shed on what these attributes should be named, and initial impl.

See #409, #369

[github-actions]

Benchmark for e64a9e4

Click to view benchmark

Test	Base	PR	%
deku_read_bits	1182.9±16.00ns	1265.0±14.41ns	+6.94%
deku_read_byte	20.2±0.11ns	20.2±0.29ns	0.00%
deku_read_enum	9.4±0.04ns	9.3±0.11ns	-1.06%
deku_read_vec	53.9±0.59ns	53.2±0.67ns	-1.30%
deku_write_bits	110.1±2.30ns	109.7±0.97ns	-0.36%
deku_write_byte	129.5±3.29ns	124.4±3.86ns	-3.94%
deku_write_enum	97.9±2.74ns	90.7±4.39ns	-7.35%
deku_write_vec	3.1±0.04µs	3.1±0.05µs	0.00%

[github-actions]

Benchmark for a800851

Click to view benchmark

Test	Base	PR	%
count_from_u8_specialize	97.8±1.13ns	97.3±0.97ns	-0.51%
count_no_specialize	2.8±0.01µs	2.4±0.01µs	-14.29%
count_specialize	99.0±0.88ns	98.2±0.70ns	-0.81%
deku_read_bits	1269.1±16.92ns	1260.8±37.53ns	-0.65%
deku_read_byte	4.9±0.06ns	4.5±0.10ns	-8.16%
deku_read_enum	4.5±0.13ns	4.1±0.02ns	-8.89%
deku_read_vec	30.9±0.46ns	31.0±0.51ns	+0.32%
deku_write_bits	238.1±4.55ns	228.7±3.91ns	-3.95%
deku_write_byte	24.0±0.28ns	24.0±0.36ns	0.00%
deku_write_enum	22.5±0.32ns	22.5±0.38ns	0.00%
deku_write_vec	513.2±10.41ns	487.1±6.74ns	-5.09%
read_all	5.2±0.04µs	4.2±0.07µs	-19.23%
read_all_bytes	5.2±0.17µs	4.2±0.06µs	-19.23%