Move exec implementations together with AST node structs

[georgeroman]

Fixes #721

It changes the following:

• Moved Executable implementations for nodes in the same files as node declarations
• Broke down some files (ast/node/declaration.rs, ast/node/iteration.rs, ...) into multiple files (one per node)

[codecov]

Codecov Report

Merging #735 into master will not change coverage.
The diff coverage is 67.64%.

@@           Coverage Diff           @@
##           master     #735   +/-   ##
=======================================
  Coverage   71.89%   71.89%           
=======================================
  Files         200      195    -5     
  Lines       14014    14014           
=======================================
  Hits        10076    10076           
  Misses       3938     3938           

Impacted Files	Coverage Δ
boa/src/exec/mod.rs	57.14% <ø> (-28.28%)	⬇️
boa/src/syntax/ast/node/break_node/tests.rs	100.00% <ø> (ø)
boa/src/syntax/ast/node/iteration/tests.rs	100.00% <ø> (ø)
boa/src/syntax/ast/node/operator/tests.rs	100.00% <ø> (ø)
boa/src/syntax/ast/node/switch/tests.rs	100.00% <ø> (ø)
boa/src/syntax/ast/node/try_node/tests.rs	100.00% <ø> (ø)
boa/src/syntax/ast/node/object/mod.rs	37.14% <43.75%> (ø)
...src/syntax/ast/node/iteration/do_while_loop/mod.rs	47.72% <47.72%> (ø)
boa/src/syntax/ast/node/statement_list/mod.rs	48.71% <60.00%> (ø)
...c/syntax/ast/node/declaration/let_decl_list/mod.rs	60.52% <60.52%> (ø)
... and 43 more

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[georgeroman]

Can I ask for a faster review on this? As a lot of files were moved, it's hard to always fix conflicts and keep this PR in sync.

[georgeroman]

Looks like an unrelated CI fail (https://github.com/boa-dev/boa/pull/735/checks?check_run_id=1198128772).