tweeks: helper functions around Env

[rakita]

• Env::boxed added
• with_env,_tx,_block added to builder
• tx(), block(), env() added to Evm with their mutable counterparts

[github-actions]

Valgrind Results:

==3985== Cachegrind, a cache and branch-prediction profiler
==3985== Copyright (C) 2002-2017, and GNU GPL'd, by Nicholas Nethercote et al.
==3985== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
==3985== Command: target/release/snailtracer
==3985== 
--3985-- warning: L3 cache found, using its data for the LL simulation.
Snailtracer Host+Interpreter benchmark (2.1s) ...            not enough samples
==3985== 
==3985== I   refs:      463,564,259
==3985== I1  misses:          3,299
==3985== LLi misses:          2,901
==3985== I1  miss rate:        0.00%
==3985== LLi miss rate:        0.00%
==3985== 
==3985== D   refs:      237,389,222  (160,565,496 rd   + 76,823,726 wr)
==3985== D1  misses:        323,294  (    183,474 rd   +    139,820 wr)
==3985== LLd misses:        137,540  (      4,060 rd   +    133,480 wr)
==3985== D1  miss rate:         0.1% (        0.1%     +        0.2%  )
==3985== LLd miss rate:         0.1% (        0.0%     +        0.2%  )
==3985== 
==3985== LL refs:           326,593  (    186,773 rd   +    139,820 wr)
==3985== LL misses:         140,441  (      6,961 rd   +    133,480 wr)
==3985== LL miss rate:          0.0% (        0.0%     +        0.2%  )