[macros] Support shrinking value transmutes

[joshlf]

In transmute!, support an #![allow(shrink)] attribute which is
invoked as follows:

transmute!(#![allow(shrink)] src);

When this attribute is provided, transmute! will permit shrinking
transmutes, in which the destination value may be smaller than the
source value.

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This PR is on branch transmute-ref-dst.

• [macros] Support shrinking value transmutes #2570
• [macros] Support shrinking reference transmutes #2487
• Generalize SizeEq to SizeFrom, support shrinking #2472